Steps to Reproduce

• Place a Tank Unit. Fill it with two buckets of water.
• Right click it with a stack of two empty buckets.
• You get a stack of two water buckets, which isn't allowed in vanilla.

Environment (please complete the following information with the version):

• Minecraft: 1.16.2
• Mod Loader: fabric

Additional context
I did it in creative, but it should also work in survival.

Thanks for reporting, fix is underway.