I'm using 19.07.8 on an Archer C7 v2.
However the workaround described by Jow in the first link re adding the multicast_to_unicast option doesn't work for me either. I'm applying it like this in
config wifi-iface 'default_radio0' option device 'radio0' option network 'lan' option mode 'ap' option key 'password' option encryption 'psk2' option ssid 'mynetwork' option multicast_to_unicast '0'
Should also say that I only just got started with openwrt, so I'm quite confident that I'm missing something...
Thanks heaps for your help!