First test case,
Cast Improved Invis : Gives Invis and Concealment effects.
Cast Invis Purge : Removes Invis and Concealment effects.
Second test case,
Cast Improved Invis : Gives Invis and Concealment effects.
Hit a nearby halfling to remove the Invis effect leaving you with just Concealment.
Cast Invis Purge : Concealment is not removed.
Third test case,
Cast Improved Invis : Gives Invis and Concealment effects.
Hit a nearby gnome (since the halfing is probably dead) to remove the Invis effect leaving you with just Concealment.
Cast Invisibility : Gives Invis effect back again.
Cast Invis Purge : Removes Invis and Concealment effects.
What this shows is that Invisibility Purge will remove the Concealment effect of Improved Invis but only if the target is invisible at the time of casting.
Now my suggestion is instead of fixing this bug, to change Invisibility Purge so it can remove the Concealment effect from Improved Invis regardless of whether the target is still invisible or not.
This would reduce the rediculous defensive capabilities of the Trickery clerics while greatly increasing the value of the spells Displacement, Ghostly Visage and Ethereal Visage since their Concealment effects would not be removed. Empty Body and Self Concealment feats would become even more valuable than they are now, along with the Concealment that may eventually be granted by Camouflage, Mass Camouflage and Vine Mine (hint hint)
.Who gets access to Invisibility Purge? Clerics and Rangers. While this would be a great boon to Rangers, it would be a bit of a double edged sword for Clerics since 90% of them have Trickery. The reason being that Invisibility Purge affects everyone in the area. Friend, foe and caster are all stripped of Invisibility. If this included the Concealment from Improved Invisibility then Rangers (hello to all 7 of you) and the non Trickery clerics (hello to all 3 of you) would have something to cheer about.