The Tank has no world.isRemote checks in onBlockActivated(), thus it runs also on the client.

(In my case this causes problems via desync of the contents if a custom item tries to cancel the processing only of the server side.)

What do you mean by that? To try if it fixes it? To try and see what breaks? To make a PR?
I did add the check, and in the situation where 6.2.4 derps up, the added check does fix it. So what exactly are you asking?

There was a small dupe with buckets before. I guess it was fixed.

Go on. Try to put the check yourself.

This is a valid bug.