Does Zipping Photos Change Their Dates?
A zip keeps the modified date but loses the created date, and the photo's own date is untouched either way.
A zip file stores each file's modified date, so that usually survives. It does not store the created date, so after unzipping, Created is set to the moment you unzipped. That alone can make a library look brand new.
The photo's own Date Taken, the one inside the picture, is not touched by zipping at all. If it was there before, it is there after. If it was missing, zipping did not cause that.
So the fix is the same as always: get the date inside the photo, where nothing can knock it off. Takeout JSON Metadata Fixer does it in one pass. It runs on your own computer and is free for your first 500 photos.