MT7628NN: Wi-Fi unstable: frequent disconnections and packet loss

I have TP-Link TL-WR842N v5 (MediaTek MT7628NN) router running OpenWrt SNAPSHOT r16952-677813c776.

My Wi-Fi setup is a simple WPA2-PSK + CCMP access point. I've faced the issue with Wi-Fi.

Wi-Fi works very unstable. Wi-Fi clients frequently disconnect and reconnect. Sometimes they connect but doesn't have an access to the network. Also there is some packet loss.

The system log is flooded with frequent disconnections and reconnections:

Tue Jun 15 10:52:15 2021 daemon.notice hostapd: wlan0: AP-STA-DISCONNECTED 80:ad:16:4f:0f:5c
Tue Jun 15 10:52:15 2021 daemon.info hostapd: wlan0: STA 80:ad:16:4f:0f:5c IEEE 802.11: authenticated
Tue Jun 15 10:52:15 2021 daemon.info hostapd: wlan0: STA 80:ad:16:4f:0f:5c IEEE 802.11: associated (aid 5)
Tue Jun 15 10:52:16 2021 daemon.notice hostapd: wlan0: AP-STA-CONNECTED 80:ad:16:4f:0f:5c
Tue Jun 15 10:52:16 2021 daemon.info hostapd: wlan0: STA 80:ad:16:4f:0f:5c WPA: pairwise key handshake completed (RSN)
Tue Jun 15 10:52:16 2021 daemon.notice hostapd: wlan0: EAPOL-4WAY-HS-COMPLETED 80:ad:16:4f:0f:5c
Tue Jun 15 10:53:22 2021 daemon.notice hostapd: wlan0: AP-STA-DISCONNECTED 80:ad:16:4f:0f:5c
Tue Jun 15 10:53:22 2021 daemon.info hostapd: wlan0: STA 80:ad:16:4f:0f:5c IEEE 802.11: authenticated
Tue Jun 15 10:53:22 2021 daemon.info hostapd: wlan0: STA 80:ad:16:4f:0f:5c IEEE 802.11: associated (aid 5)
Tue Jun 15 10:53:29 2021 daemon.info hostapd: wlan0: STA 80:ad:16:4f:0f:5c IEEE 802.11: authenticated
Tue Jun 15 10:53:29 2021 daemon.info hostapd: wlan0: STA 80:ad:16:4f:0f:5c IEEE 802.11: associated (aid 5)
Tue Jun 15 10:53:30 2021 daemon.notice hostapd: wlan0: AP-STA-CONNECTED 80:ad:16:4f:0f:5c
Tue Jun 15 10:53:30 2021 daemon.info hostapd: wlan0: STA 80:ad:16:4f:0f:5c WPA: pairwise key handshake completed (RSN)
Tue Jun 15 10:53:30 2021 daemon.notice hostapd: wlan0: EAPOL-4WAY-HS-COMPLETED 80:ad:16:4f:0f:5c
Tue Jun 15 10:54:01 2021 daemon.notice hostapd: wlan0: AP-STA-DISCONNECTED c6:e6:db:24:f7:24
Tue Jun 15 10:54:01 2021 daemon.info hostapd: wlan0: STA c6:e6:db:24:f7:24 IEEE 802.11: authenticated
Tue Jun 15 10:54:01 2021 daemon.info hostapd: wlan0: STA c6:e6:db:24:f7:24 IEEE 802.11: associated (aid 6)
Tue Jun 15 10:54:01 2021 daemon.notice hostapd: wlan0: AP-STA-CONNECTED c6:e6:db:24:f7:24
Tue Jun 15 10:54:01 2021 daemon.info hostapd: wlan0: STA c6:e6:db:24:f7:24 WPA: pairwise key handshake completed (RSN)
Tue Jun 15 10:54:01 2021 daemon.notice hostapd: wlan0: EAPOL-4WAY-HS-COMPLETED c6:e6:db:24:f7:24
Tue Jun 15 10:57:58 2021 daemon.notice hostapd: wlan0: AP-STA-DISCONNECTED 80:ad:16:4f:0f:5c
Tue Jun 15 10:57:58 2021 daemon.info hostapd: wlan0: STA 80:ad:16:4f:0f:5c IEEE 802.11: authenticated
Tue Jun 15 10:57:58 2021 daemon.info hostapd: wlan0: STA 80:ad:16:4f:0f:5c IEEE 802.11: associated (aid 5)
Tue Jun 15 10:57:59 2021 daemon.notice hostapd: wlan0: AP-STA-CONNECTED 80:ad:16:4f:0f:5c
Tue Jun 15 10:57:59 2021 daemon.info hostapd: wlan0: STA 80:ad:16:4f:0f:5c WPA: pairwise key handshake completed (RSN)
Tue Jun 15 10:57:59 2021 daemon.notice hostapd: wlan0: EAPOL-4WAY-HS-COMPLETED 80:ad:16:4f:0f:5c
Tue Jun 15 11:05:17 2021 daemon.notice hostapd: wlan0: AP-STA-DISCONNECTED c6:e6:db:24:f7:24
Tue Jun 15 11:05:17 2021 daemon.info hostapd: wlan0: STA c6:e6:db:24:f7:24 IEEE 802.11: disassociated due to inactivity
Tue Jun 15 11:05:18 2021 daemon.info hostapd: wlan0: STA c6:e6:db:24:f7:24 IEEE 802.11: deauthenticated due to inactivity (timer DEAUTH/REMOVE)
Tue Jun 15 11:06:45 2021 daemon.info hostapd: wlan0: STA c6:e6:db:24:f7:24 IEEE 802.11: authenticated
Tue Jun 15 11:06:45 2021 daemon.info hostapd: wlan0: STA c6:e6:db:24:f7:24 IEEE 802.11: associated (aid 6)
Tue Jun 15 11:06:45 2021 daemon.notice hostapd: wlan0: AP-STA-CONNECTED c6:e6:db:24:f7:24
Tue Jun 15 11:06:45 2021 daemon.info hostapd: wlan0: STA c6:e6:db:24:f7:24 WPA: pairwise key handshake completed (RSN)
Tue Jun 15 11:06:45 2021 daemon.notice hostapd: wlan0: EAPOL-4WAY-HS-COMPLETED c6:e6:db:24:f7:24

I've tried to downgrade to 19.07.7 but it didn't help much. After downgrade there is no noticeable packet loss, but there are still client disconnects (although they seems to be less frequent than on latest master) and also there is a memory leak - available memory constantly decreases during active Wi-Fi usage.

Start by reproducing the instability on the TP-Link TL-WR842N v5 with the reported MT7628NN hardware and compare the supplied hostapd logs across the OpenWrt SNAPSHOT and 19.07.7 versions. Investigate the mt76/mac80211 path implicated by the disconnects, packet loss, and decreasing available memory. Done means the affected clients remain connected, packet loss is resolved, and memory no longer continually decreases during active Wi-Fi use.