Try resetting the Samsung Camera app to its default settings by going in your Settings >> Apps menu, find the Camera app listing (since that's a pre-loaded, system app it might be separate from your installed apps), and tap on 'Force stop' and then find 'Clear data'. By tapping on Clear data that wipes the app's cache and clears any settings/configuration files, essentially returning the Camera app back to its original, first time used state.. Now start of up the Camera app and see if that made any difference.
If it's still not detecting that main, rear camera try wiping the system cache partition. The system cache is completely separate from your app caches and like any cache doesn't store any vital files so its safe to do and won't affect how you've set up your Settings or anything like that. It does require you to boot your phone up into its Recovery Mode though, instructions on that here:
https://www.hardreset.info/devices/samsung/samsung-g950f-galaxy-s8/recovery-mode/
Select the 'wipe cache partition' option and the just reboot normally. I have doubts this will make a difference in this particular instance but since you mentioned this problem started right after an update was applied, wiping the system cache is often solution to weird, glitchy issues that randomly pop up. But in doesn't hurt anything to try and can only help.
You referred to trying another app and it also wasn't detecting the rear camera, which app is it?
This last suggestion is really an iffy one but since no apps are detecting that rear camera there's a possibility it's a hardware issue. The timing seems to be too coincidental with the update being applied so it's probably a software issue but just supposition, perhaps the connector between the camera module and motherboard needs to be re-seated in the event it just worked its way loose. The iFixit site has a really helpful guides for different things in your model, this tear-down page has photos on what's involved as far as getting to the camera module. Go here (step 8):
https://www.ifixit.com/Teardown/Samsung+Galaxy+S8++Teardown/87086
But again, this is just pure speculation based on a whim.
