It is a bug, I reported it months ago. First time was on July 1 and it dropped 8 points right after I +5 min extended the workout. It happened other times too, but I never captured a screenshot. On Aug 29th I finally captured screenshots - entered the cool down of Mount Field and TSS was approximately 60, then it dropped to 23 (yes, it dropped 37 points) and I captured screenshots. A few minutes later the workout ended with a TSS of 64.
Only seen it happen when I’ve added +5 min to a cooldown - either because that triggers it, or its the only time I pay close attention to TSS during the workout.