Your phone and every other phone is supposed to do this.
When you have a sim card or esim present in your phone, this is used to identify your card in available networks.
WiFi Calling is always separated from VPN, because of several reasons. One is, your provider wants calls to be prioritized over general network traffic, so they open an extra tunnel for the voice packets.
They could not guarantee their customers undisturbed calls when there are several VPN connections in between. If there is an error in the VPN route, they can't fix it. If there is an error in the tunnel and it's clear that it is not the main connection, they have to fix the error in their own network.
You had airplane mode on but activated WiFi. When a sim is present, and your plan covers wifi calling, it automatically pings the WiFi call domain (it's the one you found in your logs). This is done independently from your settings for WiFi calling.
That's for emergency cases. If there was no cellular network, but a wifi present, it will be used for emergency calls to the police and so on.
You can only prevent this by removing the sim to interrupt Wi-Fi calling completely. You could still call emergency by disabling airplane mode, because smartphones will still connect with the first network they can reach for emergency calls.