Right. Somehow it only affects when the entry layer is IPv6 and protocol is OpenVPN with default tun-mtu 1500 as shipped by AirVPN.
I have just tried now with Wireguard on same server, same device, same wi-fi network, and the problem is not happening. Same with OpenVPN entry layer IPv4 works fine. it's just OpenVPN with entry layer IPv6 and tun-mtu set to 1500 that causes it, if I manually switch mtu to 1280 when connecting via entry layer IPv6 (OpenVPN) problem is as well fixed. Too bad my carrier (mobile sim provider) does not offer IPv6 to try, I can only try from wi-fi.