Unreliable start of multiple associations on MT7610E

I'm observing this on a C20 v5 running openwrt-21.02-rc3. I don't see it on a different hardware (e.g. MT76x2E) using the same configuration, so I assume this is an issue specific to MT7610E.

The 5GHz radio is configured as two stations connecting to the same AP in order to bridge two different VLANs (I'd like to avoid the GRETAP approach). It takes long time (minutes or hours) for the two associations to fully establish. After that happens, the connections seem stable. It looks like there is something in the initial phase that breaks the other association. The number of retries is random. When I change the configuration to use only one connection, it starts quickly.

In the log I see many messages like this (timestamps are wrong due to unsynchronized clock):

Sun Jun 20 16:03:11 2021 kern.info kernel: [ 1047.063927] wlan1: send auth to xxx (try 1/3)
Sun Jun 20 16:03:13 2021 kern.info kernel: [ 1048.498988] wlan1: send auth to xxx (try 2/3)
Sun Jun 20 16:03:13 2021 kern.info kernel: [ 1048.606968] wlan1: send auth to xxx (try 3/3)

and on the AP I see messages like this:

Fri Jun 25 10:45:23 2021 daemon.notice hostapd: wlan1-1: STA xx:xx:xx:xx:xx:xx IEEE 802.11: did not acknowledge authentication response
Fri Jun 25 10:45:24 2021 daemon.notice hostapd: wlan1-1: STA xx:xx:xx:xx:xx:xx IEEE 802.11: did not acknowledge authentication response
Fri Jun 25 10:46:12 2021 daemon.info hostapd: wlan1-1: STA xx:xx:xx:xx:xx:xx IEEE 802.11: deauthenticated due to inactivity (timer DEAUTH/REMOVE)
Fri Jun 25 10:47:13 2021 daemon.info hostapd: wlan1: STA xx:xx:xx:xx:xx:xx IEEE 802.11: authenticated
Fri Jun 25 10:47:14 2021 daemon.notice hostapd: wlan1: STA xx:xx:xx:xx:xx:xx IEEE 802.11: did not acknowledge authentication response
Fri Jun 25 10:49:36 2021 daemon.info hostapd: wlan1-1: STA xx:xx:xx:xx:xx:xx IEEE 802.11: authenticated
Fri Jun 25 10:49:36 2021 daemon.info hostapd: wlan1-1: STA xx:xx:xx:xx:xx:xx IEEE 802.11: authenticated
Fri Jun 25 10:49:36 2021 daemon.info hostapd: wlan1-1: STA xx:xx:xx:xx:xx:xx IEEE 802.11: associated (aid 2)

Start by reproducing the two-station configuration on the C20 v5 with the MT7610E and collect the client and hostapd authentication logs. Compare the behavior with the MT76x2E configuration and determine which initial association fails or interferes with the other. Done means both associations establish reliably without repeated authentication retries.