I have a very similar issue. This has been happening periodically now for a year or so. I use Samsung health synced to my strava account several times a week, mostly for walks and cycling.
Periodically the sync over to strava breaks and there are 3 scenarios that then happen.
1 - I just toggle it back on in samsung health / settings / services and all is well again
2- As above but I also have to log back into strava again - but then it's fine
3 - the login to strava doesn't work and it goes onto a full login process with 2 factor authentication. (Which i don't actually need to do with strava normally) If this happens, i know it isn't going to work. And also I can be logged in just fine to strava at the same time, but it makes no difference.
Usually a few days later it just corrects itself and its all fine again for a while.
I am currently locked out having had a walk one morning a week ago that did sync but my afternoon cycle didn't sync and the login process was back to not working - scenario 3. This is about the fourth time this has happened and its getting pretty annoying. I'm not exactly an elite athlete but I am using this to track my exercise regime following a heart attack, so it's important to me.
My take is samsung and strava know about this glitch. There is too much about this online and the fact that it just mysteriously rights itself without action by me other than constantly trying to log back in over and over again says to me there is an update happening somewhere that sorts it out. This has to be a software problem, surely.
Same issue with me using the Galaxy watch 5 to record my runs. I connect to strava through Samsung health then after a few day it disconnects and you have to re connect again. No sure if it's Samsung or Steava causing the problem.
Thanks for posting about this. Can you confirm that the workouts that are not syncing contain GPS data? Indoor workouts or activities that do not contain GPS information will not push to Strava from Samsung Health.
Another thing to check:
Check and confirm that you are using the most recent version of Strava and you have updated Samsung Health to the latest versions.
If you're still having problems after that, please go ahead and do a manual disconnect/reconnect: Disconnect Strava from Samsung Health. On the Samsung Health App, go to Menu (the 3 lines top left) > Settings (gear icon) > Connected Services. Tap Strava and confirm to Disconnect. Once disconnected, go ahead and reconnect by following the directions on Samsung Health and confirming your account information.
Please let me know if any of that helps.