So I noticed a lot of posts in the last year that were originally tagged with femsub have lost the tag. When I go into post history I notice that it the tag was never removed manually. but once you go past 1 year everything looks normal.

Ex post. hypnohub.net/post/show/59...wn_hair-corruption-curly_

I only noticed because I have femdom without femsub blocked and while searching femdom I suddenly noticed entire pages of 20+ things blocked.

from your example post I saw someone did take out femsub.

hypnohub.net/history?search=post%3A59393&show_all_tags=0

I only see open_motuh being fixed to open_mouth and that is the only removal

On that example post's Post History, with "Showing only changed tags", I see an update by "NuRho" that's blank, which I'm assuming isn't normal.

Tried to put 'femsub' back but I guess the system reacted weirdly?

This is something I've been noticing as well. I have no idea why it's happening; if it's a site error, a troll, or just people making mistakes. I have "maledom -femsub" blocked and at one point I found that half of one artist's pics were blocked for me because they had maledom, but not femsub.