The issue I ran into seems quite similar to one voiced a handful of times on the Forerunner forums starting as far back as 6 years ago. When logging a multisport activity, the specific one I did alternated between cardio and HIIT, it cumulatively logged each exercise as I went. Additionally, the Rounds in each HIIT activity stopped properly populating after Round 2 in the first HIIT session and didn't show the rest portions in sessions 2 & 3. I was able to completely delete the exercises that auto added to each cardio session of the multisport activity; however, could only edit the exercises added to each HIIT section, not delete. Screenshots below (after I made updates) to hopefully make things more clear:
HIIT Session 1:
HIIT Session 2 - everything blank (1-5) was a dupe from HIIT session 1 (excluding the rest intervals. it only replicated the sets). The last :15 seconds, I have no idea. And as you can see, no rests even though they were tracked on the watch:
HIIT Session 3:
Has anyone else run into this? Is any of it expected behavior?