I know this isn't the first post about this problem. I hope I can provide enough details to help troubleshooting this issue.
I'm running Evernote 10.15.6 on macOS Big Sur (v13.3.1). I have 70 notes, mostly text, some formatting, nothing fancy. When I search (CMD + OPT + F) and try to search for a word that is present in my notes (often in several of my notes) Evernote rarely finds the word in the note contents. It seems to only always find if the word is in the note title.
Below is a screenshot of the search function failing to find at least one document that contains the word assessment:
Just to play with it, I opened the Developer Tools (from the Help / Troubleshoot menu) and could not spot any errors in the Console or any failed requests in the Network tab. There were a couple of flagged issues, though (the same issue, twice). See this other image (in case the image disappears later, the issue is a cookie named "_ga" not being marked with the SameSite attribute). I believe this is just a Google Analytics cookie so it should not have any relation to the defect I'm experiencing.
After this I did a Force Reload and this time searching for assessment found the note in question, but it didn't fix the search. Other words continued to not be found.
I'd be happy to try more troubleshooting steps if that helps. I'm comfortable navigating the dev tools, clearing caches, local storage, inspecting values in the console, etc.