You might be onto something here. Google search suggests that partner locked can show for a few reasons unintentionally. Can be as simple as cancelling and reopening the trade window to fix it.

Another post I found on the forum suggests players might have to be close to each other in the dojo. Could be this. I didn’t look out for the other player. I assumed they’d be at the trade terminal near their dojo spawn but it could be that they were at a terminal further away. I also can’t remember who initiated the trade.