Me and others noticed this in relation to the "Terrains" tag, but it might also apply to some other tags.

1. Open https://reforger.armaplatform.com/workshop?tags=terrains.
2. Check how many mods there are.

Sometimes everything is fine, at other times there are only a couple (currently 5), and sometimes there are none.
I'm not sure if the tag search is the issue, or the mods actually temporarily lose their terrain tag, it might be the latter. This would be supported by all the currently displayed terrains being new.
Aside from that, I didn't notice any obvious pattern, there were times where it didn't show anything, a few hours later it showed them again, and then eventually mods were missing again. Can't really tell what's causing it.

This has been reproduced by several people on multiple occasions, both ingame and using the website.
Many terrains on the workshop do not have the proper "Terrains" tag, but instead something like "Island" or "Terrain", or nothing at all. This might largely just be an oversight though?