Tags are removed when shapeshifted because it wouldn't make sense that most NPCs would still recognize the character, etc.
This was actually brought up shortly after release, but modifying the design so it would allow party dialogues, etc, when in another form (but not other types of polymorph, like chicken) and continue to work properly for NPCs (all dialogues function properly that use the changes to the tag system) isn't an easy/safe task, but expect some changes in a future update.
