Working with recent Tumbleweed build (for example: 5.3.8-1-default), the
external monitor connected to laptop by HDMI port randomly entered black then
back.

Find the related information as following:
[  665.983269] [drm:intel_pipe_update_end [i915]] *ERROR* Atomic update failure
on pipe B (start=39753 end=39754) time 397 us, min 1073, max 1079, scanline
start 1069, end 1096
[20229.291090] [drm:intel_pipe_update_end [i915]] *ERROR* Atomic update failure
on pipe B (start=406108 end=406109) time 389 us, min 1073, max 1079, scanline
start 1065, end 1091
...
[22495.478664] [drm:intel_pipe_update_end [i915]] *ERROR* Atomic update failure
on pipe B (start=542080 end=542081) time 402 us, min 1073, max 1079, scanline
start 1069, end 1096
[22495.628672] [drm:intel_pipe_update_end [i915]] *ERROR* Atomic update failure
on pipe B (start=542089 end=542090) time 392 us, min 1073, max 1079, scanline
start 1069, end 1096

My laptop is Dell Inc. Precision 5530, the discrete graphics card (01:00.0
NVIDIA Corporation GP107GLM) is disabled by bbswitch.

Search some bugs filled already:
https://bugs.freedesktop.org/show_bug.cgi?id=106107
https://bugs.freedesktop.org/show_bug.cgi?id=105537