You stopped just short of telling us what Samsung device you have so we'll start with a few questions to help us help you.
What phone do you have and what update do you feel was the preluded to this behavior?
Did you have any of Samsung's safeties enabled before the update? Accidental Touch, Pocket Mode, Touch Sensitivity?
When does this happen? Just generally while using, when you're fishing it out of your pocket or bag, you never know when it happens you just see the result afterwards?
Do you use SmartLock to keep your device unlocked for extended periods? If so, what feature are you using? On Person, Location, Trusted Device?
Have you restarted your phone since the update? The way you describe things it sounds like you have, but I have to ask.
Are you using a screen protector? Are you using a phone case?
My first advice would be to enable and/or check if Accidental Touch and Pocket Mode are enabled and Touch Sensitivity is Disabled. If you find that these are set in this way, 2 enabled and 1 disabled, toggle them and then restart your device. The update may have affected one or more of these settings. Changing their status and then changing it back followed by a phone restart might reestablish their affects. A side from that, there's not much I can offer without more information. This could also be a result of a bad cache. Since Sammy took away access to the Bootloader Partition Cache Wipe you could get something like CCleaner to wipe all of your device's caches at once. Follow that with a restart and crossed fingers.
If we knew what device you had, we would look up your device and this issue to see if it's a wide spread issue or if it is being reported in other places by anyone. We could also give you specific advice and instructions.
Let us know what you find and we'll offer more advice.