Intended interaction or Bug?

So I just played a game where I encountered something odd.

I was playing Mischievous Marai shenanigans, and my opponent open attacked pulling my keg with their overwhelm unit. We go to my action to assign remaining blockers.

I thought I was being clever, using warning shot to pop the keg to allow me to block their big attacker with mirror mage, which would mess up their math and allow me to win the game on the following turn. Except I could not replace the keg. "That unit is already blocked." popup and all.

The strange thing is that there was no ghost blocker outline (which after thinking about it is probably because of their overwhelm keyword) but still this seems unintuitive to me.

If I am able to remove my own blocker at burst speed during the block assignment phase, should I not be able to just assign a new blocker if their challenged blocker is gone? The unit is now unblocked, and it is my assign blockers. You can do other stuff at burst speed which works fine with assigning blocks, like summoning a unit and using it as a new blocker with it, so why not this? Is a challenge more important?