Samsung health often does not upload to Strava until the next day. From reading up this seems to be an issue for a long time and rumours are that it is due to Samsung health pinging or contacting Strava to often and therefore getting blocked from the Strava system for a period of time and then once allowed to connect again it then uploads. This would seem to make sense as runs often upload straight away in the morning but not in the afternoon and then runs then sync sometime after midnight.
Question is Samsung if this has been a known issue for so long why the hell do you not fix it, it can't be hard and the health app gets updated all the time. Please sort this highly annoying bug out and get it updated!!