Improved Invisibility Disappearing

[Cataclysm of Iron]

Description
==========
I had an Extended Improved Invisibility concealment buff active with >15 minutes of duration left cast by my 16 Spellsword/4 Fighter. I transitioned from Skull Crags (Western Slopes) to Mount Valtheran, and the concealment effect dropped at the point when I engaged the first enemy in melee. I recast Extended Improved Invisibility, and it lasted a round, and both the invisibility and the concealment dropped completely.

Steps to reproduce
----------------------------
Cast Extended Improved Invisibility CL16. No regular versions prepared so couldn't test whether the Extended property mattered.

Expected result
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32 Rounds of Invisibility, or until first action, followed by 32 Turns of Concealment.

Actual result
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Both Concealment and Invisibility dropped after 1 Round.

[Wuthering]

Just to back you up, I posted about this a while ago:

viewtopic.php?f=12&t=27623

Something seems up with the spell. Not every time, but it's recurring.