A dropper containing a bucket of water and placed to face a cauldron dispenses the bucket, rather than filling the cauldron.

Allowing this to work would make the cauldron more useful by providing a quick refil capability.

Whether this counts as a bug or not depends on whether you consider the cauldron as a specialised container - in which case you'd expect it to be filled by a dropper since droppers do dispense into other container types. If you don't think of it as a container in the strict sense then its more of a feature request.