That sounds suspiciously like OOM: OOM kills Plasma 6?
I had that happen regardless of 1TB swap, but I was intentionally doing something to really exhaust 16GB physical RAM in that one scenario (I barely exceed 8GB normally). Do you happen to know what your RAM usage was like before the black screen?
If I understand right Plasma 6 doesn’t support Session Restore, but I’m not certain if that’s a Wayland thing or with Plasma 6 in-general.