I think I've stumbled into a few bugs prior to starting on-device Intervals / Structured workouts, but honestly the experience was so confusing at times I'm not sure how to best describe it 
My goal:
Run 6x0.5 mile intervals with 0.25 mile recovery including warm up and cool down.
Initial Conditions: Prior On device workout was 2x 9:00 with 4:00 recovery
Issue 1 - Editing the On device workout seemed to jump out of edit mode resetting any changes.
This was weird, but while I was editing my intervals (from time based to distance, from 9 minutes to 0.50... then recovery from time to distance and then repeats) the watch seemed to jump out of the editing screens resetting my partially edited changes. I suspect it was possibly due to an ANT+ sensor connecting or GPS getting a lock in the background, but maybe it was something else altogether (note, I went into this editing mode after selecting my Activity then used the menu to select Training, Structured Workout, etc...)
This is the second week in a row when I've had issues setting up a rough on-device structured interval workout. Last week I tried to edit the 2x9:00 to something else (and thought I did it) but it ended up following a partially edited or un-edited workout. I suspect I experienced this same issue at that time as well and just didn't realize which pieces were getting reverted or not saved properly. I still don't know what I'm doing wrong here or in the background is causing the behavior but will see if I continue to see weirdness here.
Issue 2 - Editing a workout after it has been selected for "Do Workout" shows all changes, but the activity follows the original workout.
Somehow in the above flow I ended up selecting "Do Workout" instead of Edit (with the original 2x9:00 efforts still shown). I suspect it was due to me hitting the "Start/Enter" button on that screen when a Sensor Connection message popped up. In my mind I assumed the enter button would help me clear the Sensor Connection pop-up and let me get to Edit, but instead it triggered "Do Workout" (more UI confusion here..)
Anyway, I realized it may have selected "Do Workout" and was unedited (due to being exited without changes in the issue 1 process) so I went back into the Training Menu and noted I had two options - I think "Edit" and "Cancel" training?
I chose Edit and proceeded to setup my intervals as desired - note this was after having done "Do Workout" with the wrong workout planned.
My edits went through as best I could tell, then I backed out and started my run thinking all was cool. At roughly the end of the first 0.5 miles I looked down to see how much was left and noticed it was actually timing a 9 minute interval and not a distance target - it seems my editing of the workout had no effect and the original plan in place when selecting "Do Workout" was being followed.
After my run is done if I review my on device Training, Intervals, Structured Repeats I see the 'new' workout I created, so the edits were saved, just not followed for the run I did.
I haven't used these features too much recently but am certain I have fairly regularly over the last ~5-7 years with FR 935 and up without seeing notable problems in these areas until these recent attempts.
