Replace laps for Garmin tcx file.
An interval workout is done with some interval (like 400m), and Garmin defaults to 1 mile laps. The result is the intervals are mixed with the recovery and averaged for the 1 mile lap.
Download the workout from the Garmin website in tcx format. Create a pyenv and download tcx.py. Do this:
python tcx.py <your_activity>.tcx > <updated_activity>.tcx
Then go back to the Garmin website and upload the tcx.
This is a quick thing with some hard-coded stuff (like 400m is the desired lap distance) that worked in one case. It likely lost some info and perhaps calculated things a bit fudgey. It's a starting point.