I find that whenever IDEInsight gets stuck (currently running mostly 10.3.3) the following rain dance helps:
Open the IDE options (Tools > Options...)
Type anything into the search field there:
Close the options
For me the F6 search works after that 100% of the time 🤷‍♂️
Because I didn't find any issue, I have now filed RSP-30365 that describes the problem of 'Preserve line ends' off still leaving the first and last line of a file with <LF> as the terminating character.