About two hours was wasted trying to figure out what became wrong and why Apple HomePod stops playing from iphone:
There are no 802.1X on this network that Learn More link points to.
The problem is that PMF capable devices can't communicate with incapable ones: https://github.com/kaloz/mwlwifi/issues/362
The router is linksys_wrt1900ac with openwrt 19.07
I've already opened bug #2890 but like to repeat it here for those who may experience same problem.
The worst thing is that luci (Interface Configuration → Wireless Security) says that 802.11w is supported by mwlwifi but in fact it works bad.