With 5.10.0-6-amd64 I don't have this message anymore; when resuming there's a few seconds wait before the screensaver login window comes up, and I get these messages in kern.log:
[145941.159088] i915 0000:00:02.0: [drm] *ERROR* Link Training Unsuccessful [145941.367056] i915 0000:00:02.0: [drm] *ERROR* Link Training Unsuccessful [145941.588149] i915 0000:00:02.0: [drm] *ERROR* Link Training Unsuccessful [145941.806389] i915 0000:00:02.0: [drm] *ERROR* Link Training Unsuccessful [145941.836878] i915 0000:00:02.0: [drm] *ERROR* CPU pipe B FIFO underrun [145942.021809] i915 0000:00:02.0: [drm] *ERROR* Link Training Unsuccessful [145942.236937] i915 0000:00:02.0: [drm] *ERROR* Link Training Unsuccessful
Perhaps that's related.