World name: Lich
NPC name: Y'shtola
Party or solo: Solo
Area and coordinates: Instanced and cutscene versions of Y'shtola, e.g. Rak'tika, Kholusia, instanced quest battles/trust system.
Steps:
The instanced model for Y'shtola seems to change at random in 5.0, switching between:
Miqo'te face #1 (default) and face #4;
Skin colour 2,5 (default) and 2,4.
Unsure about what sparks it - it's especially noticeable when you go from a cutscene to an instanced version/trust/etc. and she looks completely different from one scene to the next. It seems to be random.