This has been happening to my S9+ after the update. I think Samsung needs to send out an update. It's embarrassing to hear the person you just texted, saying hello from your pocket. Literally pocket dial. I will try the solution below.
Hi, I have the exact same problem. And Yes, also started right after a system update.
Took to Samsung. They kept for a week and didn't resolve the issue.
Did the instructions here fix your bug? Something about updates turns this feature back on.