If spending a fortune on the B-21 program ensured no adversaries believed the US invented a fundamental breakthrough in aircraft design, that would be the best reason to do it.

This is a good point.

Dropping a program for no apparent reason can be a give away that there is something else...

The Boeing X-45 program is a good example:

https://www.thedrive.com/the-war-zone/3889/the-alarming-case...

This kind of thing is highly compartmentalized. The decision makers for the budget of the B-21 program probably aren't cleared to know about a program developing an engine that ignores inertia.

Maybe. Just plain politics is more likely. Decommissioning projects like this very likely touches upon various politicians' pork projects who don't want those to go away. The military brass tried to stop the production of the M1A1 once because they had better tanks, and wanted to buy those instead, but Congress didn't because too many of their districts relied on its production.

No that's incorrect. Decisions on programs the size of the B-21 are made by the USAF Chief of Staff, senior members of the Congressional Armed Services committees, and ultimately by the President. Those people are cleared for everything related to procurement. They couldn't do their jobs otherwise.