I also observe a normal amount of deauth (not as close as originally posted here), on both the OpenWRT21 and OpenWRT22 boxes.
I have changed the settings to these:
option max_inactivity '86400'
option dtim_period '3'
option disassoc_low_ack '0'
And no skip_inactivity_poll
anywhere (don't set it to 1
).
I will keep this configuration for the time being. It seems that devices (including Apple ones) are disconnecting on their own, and this is what I prefer.