Curious - why would a (front) seat belt not retract normally, but will do so when the engine is turned off? What does engine power have to do w/ seat belt retraction?

It's a programmed comfort accommodation for folks who don't like the constant pressure of the shoulder belt when driving.

Some vehicles have constant pressure...Ford F-150. Some vehicles do not retract "normally" with the engine on or off...Mazda SUVs

The important function they all share is that they tighten upon impact...sudden deceleration