What happens if you
a) bring the drone home manually or,
b) prior to a low battery RTH starting, RTH manually and then (try to) descend or
c) if you cancel the low battery RTH (can you?) before it is above the homepoint and then try to descend?
Do you see any messages that give any clues?
There is at least one "
MA2 will not descend thread during RTH" in here
Can't ascend/descend while RTH but I do not have an
MA2 so haven't gone all the way through it...yet...... but it looks to be a bit of a frightener although it might be dealing with a slightly different situation.