Just to confirm, if you go into the Settings >> About phone menu, it's a 'clean' install yes? (.... as in no indications it's still rooted)
Are there a sizable number of apps that still indicate your phone is rooted, or just two or three or whatever?
Did you use the Google Restore from backup feature when setting your phone up after flashing it? That might be part of the problem -- the last backup being when your phone was rooted so it used some of those apps/services and their config data during the restore process. If this is the cause, you might have to just re-do everything (flash the ROM again) but this time don't use Google's Restore feature.
An important point to consider is just how did you flash the Samsung stock ROM? Did you use the suggested Odin utility? A lot of other manufacturers use different methods to flash stock ROMs, but with Samsung ROMs when you do use the Odin utility it typically does not wipe the user data partition so saved data is often left as is. That might be part of your problem, the config files from those particular apps are from back when your phone was rooted. (The internal storage media in your S8 is divided into several partitions with most of them being dedicated to the operating system and being set aside as the user data partition. Odin just rewrites operating system data into only those system partitions, the user data partition is left as is.) Anyway if you used the Odin utility, you might need to go into the Settings >> Apps menu, use the 'Clear storage button, then Uninstall. (using Clear storage should remove that app's previous config files and settings). Now install the app again from the Play Store and hopefully it's also now a 'clean' install.