Finder Quick Look Not Showing File Content

I am looking to understand why Quick Look won't show the file content in Finder sometimes.

Observations:

Files in Spotlight will nearly always show the file content.

The same file in Finder will not.

Not all files in Finder behave the same way - some will show the file content, others will not.

Opening a file, quitting the associated app, and then using Quick Look in Finder will show the file content.

MacBook Pro 2019, Big Sur 11.4

I suspect the file content in Quick Look is purged upon the Mac's reindexing, but I would like to be certain. Thank you.