I don't know what causes this but it might be Echo? When it's about to end it doesn't and every other buff applied is reapplied and persists forever. Changing a channel removes all of the buffs applied.
I'm pretty sure this is a bug that's been around for a while and the exact cause is kind of random. I had it happen to a level 5x hunter of mine after I turned in a quest and when I went back to leeching it, noticed the never ending buffs on it. The only buffs i had were MW10 and HS. I got it to go away by just going in and out of the Cash Shop quickly.
Ohh yea I just searched “buffs” and saw a bunch of posts about it. Idk why I typed “buffs don’t end” in search instead of a keyword. Big brain moment on my end.