Maybe that is the issue. Some how the server is getting the debuff order confused based on rebuffing or something. You can easily test this on the treants in SK. Buff up with 4-5 level appropriate buffs and see which ones get dispelled first.

Overwriting spells seem to bug out the client buff order the most. When fighting enchanter mobs as a monk with eipc/robe going, I've had it show the slow at the top, but the client thinks it is on the bottom and will dispel everything first and then the slow will drop. I'm pretty sure this fixes itself when you zone/camp but mid fight it's annoying to try to dispel yourself or tell someone to dispel you and it just strips buffs below it.

Dervs are going out of their way to dispel me, which is weird. That seems to have become their go-to move.

Treants cast Cancel Magic, which has a strength of 1, not 9 like the typical dragon dispel.

I'd wager that, 99% of the time, buffs appearing to be removed randomly is due to that and not buff order problems.

a buff fading (running out after its full duration) and then another being reapplied seems to bug out the buff stacking/jumping to the top

I casted 3 annuls one time on a top slotted Acid Breath from Nexona to no avail... after clicking off every single buff, annul magic worked the first time. I would imagine that sometime during the fight a fire shield faded on its own to cause some kind of buff stacking issue.

Hah, i thought this was intended.

Right. Now try it with junk buffing. Put See Invis in slot 1 and see what gets dispelled every time.

A Drachnid Recluse in Dreadlands just landed a DoT on me, then immediately dispelled his own DoT. /boggle

Mobs seem to be dispelling all the time if they have it. While I don't remember that being classic, my brother claims it happened quite a bit in Velious content, so I don't know for sure.