Holding a block in the main hand and targeting at a block causes the off hand to not work even though no block is placed because an entity is occupying that space. Easily shown by looking downwards as the player is occupying that space.
Linked issues
Comments 2
Thank you for your report!
We're tracking this issue in MC-126337, so this ticket is being resolved and linked as a duplicate.
That ticket has already been resolved as working as intended, which means this is not considered a bug and won't be fixed. Please do not leave a comment on the linked ticket.
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Duplicate of MC-126337