I’m not any more knowledgeable about this stuff than you :(, I just got an AX210 for my laptop the other day, but I don’t have a 6 GHz capable router.
It feels like it’s some kind of power saving feature or something like that. Do you actually get any faster speeds on 6 GHz?
You could try seeing if you have some kind of “roaming” or “mesh” option in your router settings. There’s a feature that’s supposed to have the router kick devices off of a connection if it thinks there’s a better one in the same mesh network. Not sure if it has any applicability to different frequencies on the same access point. Probably a dead end but you could look into it.
If it’s a fully featured router there should be tons of random options to change the power usage of the router’s wifi radios and all sorts of other stuff like that. At least on my old Asus router there were tons of options like that.