As of recent snapshots functionality was added to dispensers that allows them to fill contained bottles with water when facing water. They can also fill bottles with honey if facing a bee nest or hive which contains enough. However, they cannot fill up bottles from water-filled cauldrons, whereas players can, which is a definite issue in consistency, as players can do this.

The same thing happens when trying to fill an empty cauldron with a dispenser using water bottles, and all parts of this ticket also apply to buckets; they can't pick up water from full cauldrons nor can they fill them via dispensers.