What changed is the US President saying things like, he will encourage Putin to invade countries not spending so much on military.
What also changed is the US President threatening members of the EU militarily over greenland for example.
Reliable allies don't really do that.
(you probably do not realize the shock Denmark felt over this, that went deep and the change will not happen over night, but it will happen)
The president has repeatedly been vocal about the US leaving NATO as a possibility.
If I were the EU, I certainly wouldn't be counting on the US honoring any of its agreements and I'd be planning with the assumption that they will not join a NATO response.