Similar sporadic connected but no traffic problem bothered me for a long time.
My WiFi AP is D-Link DIR-860L B1, also mt76 driver. For me it happened not just 5G, but also 2.4G, so not DFS related.
Eventually I found the issue gone after I set these supposedly default values explicitly:
option disassoc_low_ack '1'
option skip_inactivity_poll '0'
option max_inactivity '300'
Just hope you can try it and see what happens.
BTW, you should set channel to non-DFS to eliminate the possible fault of DFS.