ath9 stopped transmitting multicast frames in Linux 5.14.0

Hello,

after upgrading Linux from 5.13.13 to 5.14.0 on my router with this wireless
device:

03:00.0 Network controller [0280]: Qualcomm Atheros AR93xx Wireless Network Adapter [168c:0030] (rev 01)
        Subsystem: Qualcomm Atheros AR93xx Wireless Network Adapter [168c:3116]
        Flags: bus master, fast devsel, latency 0, IRQ 18
        Memory at 81200000 (64-bit, non-prefetchable) [size=128K]
        Expansion ROM at 81220000 [disabled] [size=64K]
        Capabilities: [40] Power Management version 3
        Capabilities: [50] MSI: Enable- Count=1/4 Maskable+ 64bit+
        Capabilities: [70] Express Endpoint, MSI 00
        Capabilities: [100] Advanced Error Reporting
        Capabilities: [140] Virtual Channel
        Capabilities: [300] Device Serial Number 00-00-00-00-00-00-00-00
        Kernel driver in use: ath9k
        Kernel modules: ath9k

which works as a WiFi accesspoint, I noticed that WiFi clients cannot use IPv6
anymore. The problem is that the router cannot translate an IPv6 address of
the client to a MAC address. The reason is that IPv6 multicast packets used by
ICMPv6 neighbour discovery protocol do not reach the clients. In other words
the ath9k-driven device stopped transmitting multicast frames.

This can be reproduced with "ndisc6 -r 100 fe80::217:c4ff:fe4e:1552 wlan0"
command which queries (100 times) fe80::217:c4ff:fe4e:1552 address of
my client from the router. tcpdump on the ath9k device shows the queries,
but tcpdump on the client cannot see them.

The other way works correctly. I observed this with all two WiFi clients
I have (Pixel 3a phone, and a laptop with a Realtek chip). Upgrading to 5.14.1
did not help. Once I rebooted to 5.13.13, the functionality was restored.

Therefore I conclude this is regression in 5.14.0 Linux.

-- Petr