It is absolutely not just you, I have suffered from this ever since I got my Librem 13 v4 last year and did raise this in a different thread. Given the hint that it could possibly be related to Intel HD Graphics and not just the Librem, I did just try the “best” cable I could find in the house, from a Nintendo Switch, but to no avail. I have spent far too many hours trying to fix this and I am still at a loss as to what the heck it could be. After giving up some time around October last year I had hoped it would be fixed with the new coreboot version, but sadly that does not seem to be the case.
In my case I have a Dell UltraSharp UP2516D running at 2560x1440@30Hz hooked up and I am currently running both the laptop screen and the external (not that it matters, the blanking is still there with or without the laptop screen running). It is strongly correlated with CPU usage and it is only ever the external screen that goes blank.