Custom timing doesn't work consistently when applied to a duplicate workout.
After duplicating a workout (either one of the presets or an original one), custom timing cannot be easily changed for the duplicate. Changing the "Exercise duration" and "Break duration" fields have no effect, and after pressing save, those fields will revert to the values that the original workout had. Manually updating the duration of each exercise in the duplicate works SOMETIMES; it appears that some exercises will save their new values and others won't, and which one happens appears more or less random.
Edited by 12people