Thank you.
I use Joplin on Windows and Linux (sometimes also on Android).
Usually, I use the light theme, but after reading your suggestion, I changed it to dark (I'm on Linux at the moment). But it looks the same - there is no highlight founding, I have to use CTRL+F (you mentioned CTRL+L?) to find what I am looking for inside a specific note. It is really annoying.
I can't remember any changes and I have had the same (annoying) behavior for years.