I know this's been asked a lot, but I really tried everything, but still no luck to get the thumbnails back. Here's what I already did:

Regenerated Finder related .plist files;

Toggle on and off the "Show icon preview" button;

Drag things to the icon of "Get info" then deleted it;

Reinstalled Mojave;

Tried to revert back to High Sierra with Time Machined, but the backup is probably corrupted.

Reinstalled Mojave, use Migrate Assist to refill the user files;

qlmanage -r enable

Other sudo commands.

NONE of that worked! You can imagine how desperate I am right now. I'm a designer, and I need those thumbnails to identify the files.

Only when I created a new user account, it almost worked. The thumbnails loaded properly for a short period of time, but after reboot the machine they're gone.

If you can provide other solutions I'm really willing to try. Thanks!!

Updates: After testing with another admin account, I find out it might have something to do with the QuicklookSatelite process. Compare to the test account, this process just won't load when needed, I'm still trying to find out what is keeping it from working.

Updates2: Thank you! Thank you all for everything! It is indeed caused by Sketch, never thought a software can cost so much trouble. Thanks for helping me!