Why is \tl_if_in:VnTF giving a different answer depending on where it is?

Question

I'm trying to build a simple replacements dictionary using a prop structure of replacements old=new. When I'm mapping through the prop, I don't find any replacements. When I'm not (i.e., hardcoding), I do. What's going on here?

MWE

\nonstopmode
\documentclass{article}
\usepackage{xparse}

\ExplSyntaxOn

\prop_put:Nnn \l_tmpa_prop {sean} {patrick}
\msg_term:n {resolving}

\tl_set:Nn \l_tmpa_tl {hello,~sean}
\prop_map_inline:Nn \l_tmpa_prop
{
  \tl_show:N \l_tmpa_tl
  \tl_show:n {#1}
  \tl_if_in:VnTF \l_tmpa_tl {#1}
    { \msg_term:n {`#1'~found~in~`\l_tmpa_tl'} }
    { \msg_term:n {`#1'~not~found~in~`\l_tmpa_tl'} }
}

\msg_term:n {contrived}
\tl_show:N \l_tmpa_tl
\tl_show:n {sean}
\tl_if_in:VnTF \l_tmpa_tl {sean}
  { \msg_term:n {oh,~yes~it~is!} }
  { \msg_term:n {nope} }

\ExplSyntaxOff

\begin{document}
hello
\end{document}

Output

* resolving
*************************************************
> \l_tmpa_tl=hello, sean.
<recently read> }

l.18     }

> sean.
<recently read> }

l.18     }

*************************************************
* `sean' not found in `hello, sean'
*************************************************
*************************************************
* contrived
*************************************************
> \l_tmpa_tl=hello, sean.
<recently read> }

l.21     \tl_show:N \l_tmpa_tl

> sean.
<recently read> }

l.22     \tl_show:n {sean}

*************************************************
* oh, yes it is!

Answer 1

What you are forgetting here is category codes: yes, the two token lists contain the same characters, but if you use \tl_show_analysis:N you can see that they have different catcodes. This is because keys are detokenized: as the documentation says

The ⟨keys⟩ are compared on a string basis, using the same method as \str_if_eq:nn.

(We could perhaps make it explicit that they are detokenized.)

If you want to replace the value in a prop by a new one, the correct approach is to use \prop_put:Nnn, which will as-described replace the stored value with a new one when the key is the same.