The issue was first spotted by a number of users on Reddit, but reports of it haven't stopped ever since. Users have also acknowledged the fact they can't control when Wi-Fi calling does or does not happen, which results in an even bigger confusion. So, why did this sudden change of priorities happen?
It's actually pretty simple to explain: because Wi-Fi calling is a carrier-specific feature, it means any change that the carrier makes will eventually end up on your device too. It seems like T-Mobile has changed the behavior of its Wi-Fi calling feature to now be used by your iPhone only when there is little to no coverage. Previously, coverage was not really a factor and you only needed a good Wi-Fi network to make such calls.
The bad thing is neither T-Mobile nor Apple have come up with an official statement regarding the change and it went under the radar for many users, which were not pleased to discover it themselves.