To replicate:
Take a bucket of water, and try to pour it out against a double slab (any type).
The water appears for a split second, then disappears.
The water bucket appears empty, but when trying to place it a second time it pours out (unless you try using the double slab again).
If you try to pour out the bucket against a double slab more than once you lose the water.
Thank you for your report!
However, this issue is Invalid.
Your game, launcher and/or server is modified.
If you can reproduce the issue in a vanilla environment, please create a new ticket.
Any non-standard client/server/launcher build needs to be taken up with the appropriate team, not Mojang Studios or the Bugtracker.
Any plugin or resource pack issues need to be addressed to the creator of the plugin or resource pack.
If you have problems on large servers, such as The Hive and Hypixel, please contact them first as they run extensively modified server software.
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