If you create an NPC and leave the Tag field blank it automatically fills it in with NPC however in the Cast listing it doesn't drop it in with the rest of the NPCs, instead it is grouped with the players/custom grouping.

HOWEVER if you then replace the NPC tag with NPC it then groups it correctly.

My mileage differed.  :>

I created a new character attached to myself as GM and left the Tag blank.  Tag auto-filled with "NPC" and my new character was grouped under NPCs.

Which matches what is intended to happen from the notes on the Character Creation page, just as you expected:

If you do not enter a tag it will default to 'NPC' for (secondary characters for) GMs and 'player' for all other access levels.

Tried it again in a game in which I'm a Co-GM, in case that made a difference, but again, Tag auto-filled and new character went into the NPC grouping.

Can you have another go, in case it was a momentary glitch - and if not, rMail the mods the details (including game name) and the name of the NPC appearing on the PC listing and we'll have a look for you.