No longer able to find local files using the search function of Mac

I have been unable to find local pdf files using the search function on my Mac since the OS has been updated lately to big sur 11.5.2 (maybe even some earlier versions but I didn't pay attention to). This became very clear after I recently migrated a lot of files from local dropbox folders to google drive's local folders. Whatever the reason is, this makes the search function much, much less desirable than it has always been till recently.

What's weird is that if I navigated into the folder where that pdf file has been stored and opened & closed it and searched the file again, it can be found. In other words the search function seems now to have become so lazy as to only look into cache for the result, without visiting the whole document directory.

Really wish the Apple engineers can fix this problem at next upgrade of the OS. (or perhaps people have other advice on how to fix it myself?)

Thanks!