Latex repeat equation numbering (unwanted)

Question

I'm using the equation environment in my latex document. All went ok until half a chapter when I find the two equation have the same equation number (3.44). Why could this happens? Moreover, from that point to the end of the document, all the equations that are within the same subsection/section have the same numbering.

This is the crucial point of the document: At first

\begin{equation}
\mathbf { L } \hat { \mathbf { U } } _ { \mathcal { K } } = \hat { \mathbf { U } } _ { \mathcal { K } } \mathbf { C } _ { \mathcal { K } }
\end{equation}

that currently follow the equation numbering order and it is (3.44). Then I insert two equation that must not to be numbered, thus I used the tag environment:

\begin{equation}
\quad &\min _ { \mathbf { L } \in \mathbb { R } ^ { N \times N } , \mathrm { C } _ { \mathcal { K } } \in \mathbb { R } ^ { K \times K } } f ( \mathbf { L } , \mathbf { Y } , \hat { \mathbf { S } } )
\tag{$\mathcal{P}_{L}$}
\end{equation}
\begin{equation}
\notag
\quad \quad & & & & \left. \begin{array} { c l } { \text { s.t. } } & { \mathbf { L } \in \mathcal { L } , \operatorname { tr } ( \mathbf { L } ) = p } \\ { } & { \mathbf { L } \hat { \mathbf { U } } _ { \mathcal { K } } = \hat { \mathbf { U } } _ { \mathcal { K } } \mathbf { C } _ { \mathcal { K } } , \mathbf { C } _ { \mathcal { K } \succeq } \mathbf { 0 } } \end{array} \right\} \triangleq \mathcal { X } \left( \hat { \mathbf { U } } _ { \mathcal { K } } \right)
\end{equation}

\begin{equation}
\quad &\min _ { \mathbf { L } \in \mathbb { R } ^ { N \times N } \atop \mathrm { C } _ { \mathcal { K } } \in \mathbb { R } ^ { K \times K } } f _ { 1 } ( \mathbf { L } , \mathbf { Y } , \mu ) \triangleq \mathrm { TV } ( \mathbf { L } , \mathbf { Y } ) + \mu \| \mathbf { L } \| _ { F } ^ { 2 }
\tag{$\mathcal{P}_{L_{1}}$}
\end{equation}
\begin{equation}
\notag
 \left. \begin{array} { c l } { \text { s.t. } } & \quad \left( \mathbf { L } , \mathbf { C } _ { \mathcal { K } } \right) \in \mathcal { X } \left( \hat { \mathbf { U } } _ { \mathcal { K } } \right)
\end{equation}

that has no equation number but the tag filled. Then when writing

\begin{equation}
\operatorname { tr } \left( \mathbf { Y } ^ { T } \mathbf { L Y } \right) = \operatorname { tr } \left( \mathbf { S } _ { \mathcal { K } } ^ { T } \mathbf { \Lambda } _ { \mathcal { K } } \mathbf { S } _ { \mathcal { K } } \right) = \operatorname { tr } \left( \hat { \mathbf { S } } _ { \mathcal { K } } ^ { T } \mathbf { C } _ { \mathcal { K } } \hat { \mathbf { S } } _ { \mathcal { K } } \right)
\end{equation}

it has still equation number 3.44. Why?

EDIT: I wrote the two consecutive equation environemt to obtain the following effect

Answer 1

You get a huge number of errors from that input. Whatever happens next is not reliable until you fix the errors.

Here's a fixed version:

\documentclass{article}
\usepackage{amsmath,amssymb}

\DeclareMathOperator{\tr}{tr}

\begin{document}

\begin{align}
&\min_{\substack{
  \mathbf{L}\in\mathbb{R}^{N\times N} \\
  \mathbf{C}_{\mathcal{K}}\in\mathbb{R}^{K\times K}
}} f(\mathbf{L},\mathbf{Y},\hat{\mathbf{S}})
\tag{$\mathcal{P}_{L}$}
\\
\notag
&\quad
\left.\begin{array}{@{}ll@{}}
\text{s.t.} & \mathbf{L}\in\mathcal{L},\tr(\mathbf{L})=p\\
            & \mathbf{L}\hat{\mathbf{U}}_{\mathcal{K}}=
              \hat{\mathbf{U}}_{\mathcal{K}}\mathbf{C}_{\mathcal{K}},
              \mathbf{C}_{\mathcal{K}\succeq\mathbf{0}}
\end{array}\right\}
\triangleq\mathcal{X}(\hat{\mathbf{U}}_{\mathcal{K}})
\\[2ex]
&\min_{\substack{
  \mathbf{L}\in\mathbb{R}^{N\times N} \\
  \mathbf{C}_{\mathcal{K}}\in\mathbb{R}^{K\times K}
}} f_{1}(\mathbf{L},\mathbf{Y},\mu)\triangleq
  \mathrm{TV}(\mathbf{L},\mathbf{Y})+\mu\|\mathbf{L}\|_{F}^{2}
\tag{$\mathcal{P}_{L_{1}}$}
\\
\notag
&\quad
\,\begin{array}{@{}ll@{}}
\text{s.t.} & (\mathbf{L},\mathbf{C}_{\mathcal{K}})\in\mathcal{X}
              (\hat{\mathbf{U}}_{\mathcal{K}})
\end{array}
\end{align}

\begin{equation}
\tr(\mathbf{Y}^{T}\mathbf{LY})=
\tr(\mathbf{S}_{\mathcal{K}}^{T}
    \mathbf{\Lambda}_{\mathcal{K}}
    \mathbf{S}_{\mathcal{K}})=
\tr(\hat{\mathbf{S}}_{\mathcal{K}}^{T}
    \mathbf{C}_{\mathcal{K}}
    \hat{\mathbf{S}}_{\mathcal{K}})
\end{equation}

\end{document}

With different alignment. A couple of tricks are needed to move s.t to the left with the brace on the right hand side.

\documentclass{article}
\usepackage{amsmath,amssymb}

\DeclareMathOperator{\tr}{tr}

\begin{document}

\begin{align}
\hphantom{\text{s.t.}\quad}
&\min_{\substack{
  \mathbf{L}\in\mathbb{R}^{N\times N} \\
  \mathbf{C}_{\mathcal{K}}\in\mathbb{R}^{K\times K}
}} f(\mathbf{L},\mathbf{Y},\hat{\mathbf{S}})
\tag{$\mathcal{P}_{L}$}
\\
\notag
&\left.\kern-\nulldelimiterspace
\begin{array}{@{}l@{}}
 \makebox[0pt][r]{s.t.\quad}
 \mathbf{L}\in\mathcal{L},\tr(\mathbf{L})=p\\
 \mathbf{L}\hat{\mathbf{U}}_{\mathcal{K}}=
   \hat{\mathbf{U}}_{\mathcal{K}}\mathbf{C}_{\mathcal{K}},
   \mathbf{C}_{\mathcal{K}\succeq\mathbf{0}}
\end{array}\right\}
\triangleq\mathcal{X}(\hat{\mathbf{U}}_{\mathcal{K}})
\\[2ex]
&\min_{\substack{
  \mathbf{L}\in\mathbb{R}^{N\times N} \\
  \mathbf{C}_{\mathcal{K}}\in\mathbb{R}^{K\times K}
}} f_{1}(\mathbf{L},\mathbf{Y},\mu)\triangleq
  \mathrm{TV}(\mathbf{L},\mathbf{Y})+\mu\|\mathbf{L}\|_{F}^{2}
\tag{$\mathcal{P}_{L_{1}}$}
\\
\notag
&\makebox[0pt][r]{s.t.\quad}
 (\mathbf{L},\mathbf{C}_{\mathcal{K}})\in\mathcal{X}
 (\hat{\mathbf{U}}_{\mathcal{K}})
\end{align}

\begin{equation}
\tr(\mathbf{Y}^{T}\mathbf{LY})=
\tr(\mathbf{S}_{\mathcal{K}}^{T}
    \mathbf{\Lambda}_{\mathcal{K}}
    \mathbf{S}_{\mathcal{K}})=
\tr(\hat{\mathbf{S}}_{\mathcal{K}}^{T}
    \mathbf{C}_{\mathcal{K}}
    \hat{\mathbf{S}}_{\mathcal{K}})
\end{equation}

\end{document}