The problem is due to a bug in the PokerStars client update (I believe it only impacts macOS)
On Windows and previously on macOS, hand history files were named as such:
HH20211214 [TABLE NAME] [GAME] [STAKES].txt
Now they are just named:
Two third of the filename is missing and so is the file extension.
The tournament summaries are still being named correctly; hence, why they still import.
I would contact PokerStars and notify them of this bug and hopefully they fix it ASAP.