Update: I tried using the MultiMC launcher and that saves the region files fine, so this may be a launcher issue rather than a snapshot issue.

I made a launcher profile for 18w50a because I didn't want to update a world to 19w02a. I have launched with this profile several times and it still says "Download" in the launcher. Then when I play, it doesn't save the region files. The next time I log in, the player location and inventory are where I logged out, but all the blocks I placed in my prior session are gone. Looking at the save files, everything in the playerdata and data folders have updated timestamps, but nothing in the region folder does.