Metasounds cease functionality if called to Play with a different start time other than 0. This extends to the Metasound editor where pressing play to preview the sound also ceases to function.
Metasound performs as expected even after exiting PIE.
The Metasound stops playing, and will not play again even when attempting to play it within the Metasound Editor.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-136580 in the post.