In Flux 8.0.13 build #98, Flow Chart Action blocks until the child workflow completes. By "completes", that means the workflow runs to completion and is no longer on the engine.
However, sometime after Flux 8.0.13 build #98 (confirmed in build #2662), Flow Chart Action continues to the next action when its child workflow enters the FAILED state, as though the child workflow had completed.
The bug fix will revert to the build #98 behavior. A child flow chart must run to completion (that is, no longer be on the engine) before the parent flow chart can continue. (This assumes the Flow Chart Action is running synchronously. The bug does not impact Flow Chart Action when the Asynchronous flag is enabled.)
This bug will be fixed in Flux 8.0.15 and 8.1.2. (And, of course, the forthcoming Flux 8.2.0.)