Humans, to their credit, have mastered the mass-to-energy conversion equation with explosive results. I suspect someone else has mastered the space-time conversion and substituted propulsion with space-time manipulation by means yet unknown to humans.

This possible solution to UAP realities would satisfy the peculiar flight characteristics mentioned below as well as a few more.
The technology of space-time travel is not without glitches.
Even lightning can throw a monkey wrench into the works.
UAPs have been rumored to crash, although I have never seen that.

I do not buy into the notion that UAPs are a navigational danger to our aircraft. Such an event is more rare than rumored crashes of UAPs on their own. If one could manipulate space time flight they wouldn't just miss something by a mile, they could miss something by hours.