The bug
When holding any seed in the right hand and bone meal in the off hand and right clicking on any planted seed, bone meal cannot be used on that seed.
Related issues
Comments

What item (if any) were you holding in your main hand, and which blocks were you clicking on?
in my main hand i was halding carrot in of hand i was holding bonemeal and was clicking right click on planted carrots
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the bug happens when holding any seed in the right hand and bone meal in the off hand and right clicking on any planted seed, tried on 1.14.3 and 1.14.4-pre4

Fixed in 20w12a.

No, this is still an issue in 20w12a.

Aah. My bad it work when you have only bonemeal in offhand and when mainhand is empty.

If you are holding a block in your main hand, then you can't perform off-hand actions even tho they are valid.
Example:
Block in main hand, torch in off-hand. Aim directly at the block beneath you.
The torch should be able to be placed because it is the only valid action.
But it wasn't placed.
Also true for other right-click actions such as bone meal
If you go very far back (~1.9, 1.10), you can see that this wasn't the behaviour when dual-wielding was first introduced.
This bug is still in 1.16.1, has been lingering for a very long time now from what it seems.

Thank you for your report!
We're tracking this issue as 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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I am not able to reproduce the bug. Bonemeal seems to work fine while using it in the off handΒ