A bit more info to add here just in case it’s useful to anyone.
My first (and so far only) occurrence of this was on Saturday, 12 Jan. Prior to this, I had last used the computer on Thursday afternoon. My work Thursday was interrupted, so I never shut down and it went to sleep on its own. When I woke it (Saturday morning, I worked on my Macbook inbetween though, not that lucky…) the new white border was there. This looked very much like the pic the OP posted. What you can’t see clearly on that pic though (and I’m not sure if Joseph had the same) is that the whole screen’s resolution wasn’t quite 100%. Even in the center of the screen things appeared to be a little lighter and more grainy than usual, this effect was worse the closer you got to the edges and faded into the border that you see in the pic.
We had a heavy electrical storm on Thursday evening, so my first thought was that there must have been some hardware damage, although in retrospect this is unlikely (electrical storm damage is usually a bit more all-or-nothing and other than the display everything seemed to be 100% normal).
I then proceeded to try and troubleshoot. Ran some diagnostics, reset NVRAM, restarted the computer several times using different permutations, tried booting into recovery mode (border was also there in recovery mode) etc. Nothing had any effect, but I noticed that the border (and the effect through the rest of the screen) was gradually disappearing. After about an hour the border was much smaller (interestingly it also started fading away from the edge, wish I had taken a pic of this, but at this point it was a faded border about 5mm from the edges and around 10mm thick). At this point I couldn’t really notice any quality issues in the center of the screen and the border was small enough that it really didn’t bother me that much anymore so I decided to get on with my life and ignore it while it disappears. After about 3-4 hours I couldn’t see any remnants of the issue anymore. To the naked eye, the display was perfect.
Since then I’ve experimented with (a) a prolonged shut down (>12 hours) period and start to see if it recurs (it did not) and (b) a prolonged sleep (>12 hours) period and wake up and see if it recurs (it did not). If an opportunity arises to test a longer sleep period, I’ll give that a shot too, although the ±36 hours sleep that it had prior to the first occurrence was unusual and would require a bit more interruption to my normal routine.
So unfortunately still no idea what the cause is, but mine appears to be fine now and if you’re having the same issue, hopefully yours will be too