It is impossible to place blocks on cauldrons with level 1, 2, or 3, other than by sneaking. It works fine with empty cauldrons.
How to reproduce:
-Place a cauldron
-Fill it with any amount of water.
-Attempt to place a block while not crouching.
Related issues
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Thank you for your report!
However, we are already tracking this issue, and this report is a Duplicate of MC-133691.
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