Hi Pauli,
I have seen that before on a previous Canary Build. Other users reported the same issue.
I think I restored an older backup to get rid of it.
On the last 2 canary Builds I had a different issue where the Boot menu did not show up at all and got a black screen.
In this case the bug is created by one experimental feature that MS enabled on 2 machines.
Only happens if Canary is the default boot system.
For this one I found the culprit and disabled it, and everything is back to normal. The effect may be different depending on the display adapters configuration.