This is the second time this has happened to me, managed to grab a photo this time.
Both times (I think at least) I was grabbing the phone to use it and found the lock screen looked like the below photo. Almost like I accidentally grabbed the phone in some way which triggered this (or it registered input while in my pocket).
I would consider this a very nasty bug - I tried a few combinations this time to “escape” the behavior or trigger a UI but nothing worked. My concern here is that if this happens at the wrong time, I (or someone else) can’t activate it for an emergency call.
Just for clarity did the Soft reset with Power + Volume Down for >10 - 15 seconds (suggested in the link) fix / ameliorate the issue? This is a Samsung “special feature” to stop undesired behaviour.
I had exactly the same intermittant issue with an old Samsung Galaxy S5 that i used to have eOS on. I replaced it with a Samsung Galaxy S9 a few years back which has never shown this problem, so even though Samsung seems to be the common element here, it doesn’t seem to crop up on all Samsung devices.
I had the exact same problem for the first time a few days ago (on a Samsung S7). I tried the “force reboot” thing but it didn’t work at first, but maybe I did it wrong because I retried later and it worked.