Environment:

1.21.3

NeoForge 21.3.33-beta
Fabric Loader 0.16.9
Fabric API 0.107.3+1.21.3
ImmediatelyFast 1.3.2+1.21.3
Ayame PaperDoll 4.1.1

1.21.1

NeoForge 21.1.80
Fabric Loader 0.16.9
Fabric API 0.108.0+1.21.1
ImmediatelyFast 1.3.2+1.21.1
Ayame PaperDoll 4.0.2

When Ayame PaperDoll's "Mirrored" option is set to true and used with ImmediatelyFast, the player cape in Ayame PaperDoll renders incorrectly. This issue can be reproduced in Fabric and NeoForge from Minecraft 1.21 to 1.21.3.

Without ImmediatelyFast:

ImmediatelyFast Fabric 1.21.3

ImmediatelyFast NeoForge 1.21.3

ImmediatelyFast Fabric 1.21.1

ImmediatelyFast NeoForge 1.21.1

It seems to have been fixed in v1.3.3, thanks : )