With IntelliJ 2021.1, I have noticed the search of files or words across a project is a bit flakey.
So if you paste a word to search with cmd + shift + f, it struggles to find all the words, and if you enter it from beginning it is better but still problematic. Even double shift is struggling to find files.
I didn't notice such issues with previous version.
It's a known bug.
Please add -Didea.indexes.clear.non.indexable.file.data=false into Help | Edit Custom VM Options and invoke File | Invalidate Caches/Restart.. | Invalidate and Restart action as a workaround.