While testing a new feature in my JSON addon for Tinkers', I noticed that the fluid bucket does not have a fluid handler registered.
This is due to the fact that you extend BucketItem with FlexBucketItem, but Forge only exposes its capability if the class is exactly BucketItem. It should be as simple a fix as overriding the method to use the forge capability handler.

As an aside, it would be nice if bucket items automatically set the container to minecraft:bucket if unset, along with the max stack size to 1 if unset. Not sure if your setup makes that reasonable to do easily or not; just would save some confusion with forgetting to set those values.
Experienced this bug on 1.20.1 on Json Things 0.9.9, have not verified the Neo source on 1.21 but I suspect the issue persists unless Neo changed their BucketItem patch.
While testing a new feature in my JSON addon for Tinkers', I noticed that the fluid bucket does not have a fluid handler registered.
This is due to the fact that you extend
BucketItemwithFlexBucketItem, but Forge only exposes its capability if the class is exactlyBucketItem. It should be as simple a fix as overriding the method to use the forge capability handler.As an aside, it would be nice if bucket items automatically set the container to
minecraft:bucketif unset, along with the max stack size to 1 if unset. Not sure if your setup makes that reasonable to do easily or not; just would save some confusion with forgetting to set those values.Experienced this bug on 1.20.1 on Json Things 0.9.9, have not verified the Neo source on 1.21 but I suspect the issue persists unless Neo changed their
BucketItempatch.