I've seen this same behaviour with IOS messaging ten years ago; I would travel between countries with roaming enabled and every time I changed countries and turned on my iPhone, iMessage would be waiting for activation.

Once spent 5 hours on the phone with an iMessage developer in Ireland helping them debug the issue.

At that time, we didn't have eSIM so I ended up with an Android phone for roaming and my iPhone for home country.

Many months later I got an update from Apple. It was something to do with activation. iOS used to send a hidden SMS to a server in the UK and sometimes while roaming it would time out.