Yes that seems to be what's happening. The question is - should that be happening?
D2 is an old game and had lots of never-fixed bugs - it seems pretty standard to me that if someone casts a skill that affects stats then you run stuff like gear-check routines to pick up the effects. I think Nitz expected that too - I got my expectations from the comments in the "Natalya The Insomniac" Electrofield Assassin guide where Nitz said this (post #12 of that thread):
Dexterity: Enough to equip your gear (without TForm)
Why? I would go for enough to wear with Tform i.e. nothing. Just cast it when you die and you can wear anything.
Since dex is useless for a caster sin except to wear gear, T.Form becomes questionable if you need to have the base dex anyway. +1 skills and some life for 30 points is a bit mediocre.
I guess you could try to always remember to do a pointless weapon-swap every time you join a game or re-buff Tiefling, but... pain