Odd (nasty) problem with relsize for quotes and linespread in preceding paragraph

Question

On the last few metres of typesetting a book I encountered a very nasty issue which I luckily managed to isolate in a MWE. The problem is that blockquotes patched with relsize to have a slightly reduced text size (next smaller "tex size" is too small for book format imho) leads to a diminished linespread in the preceding paragraph - if there is no empty line in the code before \begin{quote}.

Following MWE has more pronounced values to show the effect. Note the difference in the empty line in the code and the changed outcome for paragraph 1 and 2. With relsize of .9 (as in the real case) it's hardly visible.

Does this mean that the patching affects the preceding paragraph somehow? Seems like a bug since it only affects linespread and not text size.

Occurred with Texlive/Xetex but Overleaf/Latex produces the same problem. The problem seems to relate to the relsize part since I get a similar result with setspace/setstretch.

Some feedback on what went wrong would be appreciated, since I am a uneasy that issues like this might also happen in other instances, where I might not have noticed them yet.

\documentclass[fontsize=11.5pt,twoside]{scrreprt} 
\usepackage{blindtext}
\usepackage{etoolbox}
\usepackage{relsize}

\linespread{1.8} %for effect

\AtBeginEnvironment{quote}{\relscale{.3}} %smaller than real, also for effect

\begin{document}
    \subsection{problem}
\blindtext 

    \begin{quote}
\blindtext
    \end{quote}
\blindtext 
        \begin{quote}
    \blindtext
    \end{quote}
\blindtext

\end{document}

Answer 1

Your patch is wrong. You are adding the \relsize actually before the quote.

Try out

\AtBeginEnvironment{quote}{\relscale{.3}xxxxx}

to see what I mean.

Better use

\appto\quote{\relscale{.3}}

Answer 2

You need to end a paragraph when changing font size. If you don't, the \baselineskip which determines the line spacing will be incorrect for one or more sizes. This is because certain settings, including this one, are applied to paragraphs in their entirety and only when the paragraph ends. So the \baselineskip for all parts of a paragraph is determined by the \baselineskip currently active when the paragraph ends.

The problem can, therefore, be avoided by inserting some paragraph breaks. If you do not want actual paragraph breaks, use \noindent to avoid indentation of the first line.

\documentclass[fontsize=11.5pt,twoside]{scrreprt}% note that this means 12pt font is used
\usepackage{blindtext}
\usepackage{etoolbox}
\usepackage{relsize}

\linespread{1.8} %for effect - note that you really, really, really shouldn't do this like this

\AtBeginEnvironment{quote}{\relscale{.3}} %smaller than real, also for effect - note that the resulting sizes are larger than provided here

\begin{document}
\subsection{problem}
\blindtext 

\begin{quote}
  \blindtext
\end{quote}

\noindent\blindtext 

\begin{quote}
  \blindtext
\end{quote}

\noindent\blindtext

\end{document}

If you really want to do things like this, you should read the relevant parts of KOMA's manual and use setspace as it advised. Moreover, you are requesting font sizes which aren't available. See the console or log for details and consider using scalable fonts. Any of the following will get you such fonts

\usepackage[T1]{fontenc}
\usepackage{lmodern}
\usepackage{cfr-lm}
\usepackage{<other type1 font package>}