Version Info

• Minecraft, 1.21.1
• Fusion, 1.2.10

What mod loader are you using?: NeoForge

Are you using OptiFine: No

Description of the Bug

SpriteLoaderMixin (and presumably other mixins) use @Inject where @ModifyReturnValue or @WrapMethod from MixinExtras would perhaps be more appropriate. I believe this is causing this problem, as I have explained in that issue's discussion, though admittedly this is only my first instinct and the problem may be on my library's end. Either way it would be good future-proofing to use MixinExtras.

SpriteLoaderMixin (and presumably other mixins) use @Inject where @ModifyReturnValue or @WrapMethod from MixinExtras would perhaps be more appropriate.

@Inject just injects the method at some point in the target method. As long as the injection does not cancel the method, that is completely fine and has no compatibility issues. MixinExtras does not have anything to replace regular @Injects as there is no reason to.

The issue seems to be caused by SpriteLoaderMixin from Petrolpark Library. The mixin overwrites the set of metadata section serializers given to SpriteLoader#loadAndStitch with OffGridTilingMetadataSection#DEFAULT_METADATA_SERIALIZERS_WITH_OGT which contains just OffGridTilingMetadataSection serializer and the vanilla animation metadata serializer.
This means all other metadata, like those from other mods such as Fusion, will be ignored with Petrolpark Library installed.

Fusion adds a new metadata section for specifying Fusion texture types and properties. Since the metadata is now ignored, none of the textures using Fusion will work.

Easiest way to add a texture metadata serializer is simply adding your metadata serializer to SpriteLoader#DEFAULT_METADATA_SECTIONS such that it is compatible with other mods adding metadata sections.
I do the same in Fusion here:

Yeah, makes sense. What I said was just my guess from five minutes of looking. I'll implement that change; thank you for the help.