Portrait editing mode – odd animation twitch on particular poses

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Date & Time: any
Frequency: always when editing
World name: -
Character name: multiple
Retainer name: -
NPC name: -
Monster name: -
Class/Level: any
Party or solo: solo
In-game time: -
Area and coordinates: Edit Portrait menu
Housing: -

Steps:
1. Go into the "Edit Portrait" menu via gearsets, adventurer plate, etc.

2. Switch to the character editing tab and set the pose to "Standing 8" (battle stance) or "Standing 9" (non-battle pose with weapon drawn).

3. Observe the character's face. Every few seconds their eyebrows and possibly other parts of their face (depending on the exact pose, which varies by job) will twitch.

This does not happen when actually using the pose with the character "for real" in the game, only when using it in the portrait editor.

I have tested this on male Viera and male Au Ra characters, so it it likely happening to all character models.