The Margaret outfit is the one where she's half-size after drinking too much of the tea. I'd replay that scene to see if it triggers it again.
The chapter should happen after you play through Gavin Epilogue: Home At Last and choose "Just go furry sometimes." It looks like you already did that if you have the subsequent CGs, but sometimes it can trigger the second time. I'd suggest clicking through quickly instead of holding down skip so it has time to register the trigger.
