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Date & Time: Since 6.1
Frequency: Always, but it seems to happen "randomly"? It's always the same plates/gearsets, but I can't determine what makes the message trigger.
World name: Mateus
Character name: Mayah Amariyo
Class/Level: Gathering jobs, Crafting jobs, Warrior and Dark Knight and Gunbreaker (Paladin, while sharing the same plate, doesn't get the error message?), Reaper and Monk (even though they share a plate with Dragoon, which doesn't display the error message when I switch to it either??)
Party or solo: Solo
Area and coordinates: Every sanctuary
Steps:
1. Be a female miqo'te
2. Not use any fantasia in years
3. Never make a glamour plate using other races or gender gear
4. Make a glamour plate with one or more of the following pieces:
- Rainbow Hydrangea Corsage
- Spriggan Jacket
- Linen Halfglove
- Scion Adventurer's Bottoms
- Spriggan Boots
- Casual Jacket
- Lucian Prince's Fingerless Glove
- Lucian Prince's Bottoms
- Calfskin Rider's Shoes
- Augmented Rathalos Helm [F]
- Augmented Rathalos Mail [F]
- Augmented Rathalos Vambraces[F]
- Scion Traveler's Trousers
- Augmented Rathalos Greaves [F]
- Resshi Hachigane
- Miqo'te Separates
- Eastern Lady Errant's Gloves
- Skyworker's Bottoms
- Rebel Boots

- Probably more, but those are the potential culprit pieces. The most confusing is the first few ones, since none of these items (from Rainbow Hydrangea Corsage to Spriggan Boots) were ever genderlocked from what I can recall (though the Scion Adventurer's Bottoms do change when equipped on a different gender)

5. Apply the glamour plate
6.
7. The plate applies normally anyway

Let me know if more info is needed, that's all I have for now!