I thought this was well understood, and even expected by the models, since a warming planet brings more precipitation in the air which then falls as snow on the poles.

So the Antarctic is actually gaining mass on the higher ground, and the Arctic during the winter months also.

The contrast is that during the warmer summer months, the Arctic is losing a lot more than it gains over the year, so the global balance is negative - which adds to sea level rises.

The other odd thing I never considered, but was (I think here?) mentioned, is that as the mass of the Arctic decreases, it also presses down less on the mantle, and so lifts higher out of the ocean, also displacing more sea.

I'm pretty sure there's a good explanation of this by Potholer54 on Youtube.