Notice the timing of the flash
Roku says auto-adjust display refresh rate can make the TV briefly flash black when a programme starts or stops because the television changes modes. Apps with automatic video previews can trigger more frequent transitions.
A brief transition followed by stable playback differs from repeated stalls during a scene. Note exactly when it happens and whether the picture returns promptly.
Make one setting comparison
Inspect Auto-adjust display refresh rate on the compatible Roku player and record its state. Compare the same programme with the relevant output choice changed, keeping the TV and source fixed.
Disabling a transition can change the timing used for the programme. Judge that tradeoff explicitly rather than calling a faster start a source-quality improvement.
Know when this explanation is insufficient
A persistent blank, corrupted picture or failure in system menus needs further display-route investigation. Do not label every black screen normal merely because refresh switching exists.
Report the player, app, auto-adjust state and whether the issue appears at start, stop or during playback. That scope is more useful than a generic claim that Roku loses the stream.
Questions answered
Can a short black flash be normal with auto-adjust enabled?
Roku documents it as a possible television mode-change effect.
Does this explain a picture that never returns?
No. A persistent failure needs further investigation.
Sources & further reading
Official references for the subjects discussed. Service terms and device software can change.