I suck at physbones and animations, so here's an AI summary that fixed it for me:
I inspected the Milltina sweatshirt prefabs, scene overrides, PhysBones, controllers, and animation clips.
I found the PhysBone endpoint on Grap_phys/Grapb_001 was {0,0,0} in all 10 Basic/Big100 gimmick prefabs. I changed its Y endpoint to 0.02, which restored the PhysBone topology and made the shirt respond.
I changed the Modular Avatar animator merge from absolute paths to relative paths and corrected the animation bindings so they target Shirt, Shirt_Small_1, and Shirt_Small_2 correctly.
I kept Milltina’s maxStretch at 6, including the scene override.
I changed Unity’s Play Mode reload settings back to normal reloads, which stopped the crash occurring when entering Play.
I traced the Up4 problem to the animation clips. The controller uses Shirt_Up_Stretch as normalized time, but the clips’ actual keys ended at 0.75. Setting only m_StopTime to 1 was not enough.
I changed the final Up4 and Up4.5 keys in both Shirt_Up.anim and Shirt_Up_B.anim to time 1, restored the unrelated curve edits, reimported the clips, and verified the result through Gesture Manager.
