When you fill the cauldron with water and you right click with a shulker box , the water in the cauldron disappear. it only effect in minecraft version 1.16.5.(Not in 1.17)
MC-230919
Cauldron Bugs
Resolved
Duplicate
0
1.16.5 is not supported. Bug reports are only accepted for the latest version (1.17) and latest snapshot.
In this case, the bug has already been fixed in 1.17, see MC-126997.