What steps will reproduce the problem?
1. Usually going into an instance with others does it, but not always, even when you're with the same group with the same buffs, so it isn't a case of another buff overwriting the Firestone/Spellstone.  Seems to happen often after I've died and then rezzed either by zoning in or being resurrected by another player.  Maybe something about the buffs going away while dead and not being applied correctly again by DoTimer upon ressurection?  I can't say for sure.

What is the expected output? What do you see instead?
When I'm by myself, I almost always see the weapon buff, but often (about 70% of the time) if I'm in a group it disappears and reappears seemingly randomly.  Sometimes dying makes it disappear, sometimes zoning in or out (but only instances so far)

What version of the product are you using?
v4.1

Do you have an error log of what happened?
It generates no error log.