This has happened quite a few times and seems to be when the diagram is updated from the server. I am not sure if the execution bar remains in place or the lifeline remains in place, but it always seemed that the execution bars were to the right of the lifeline.
If the lifeline is moved even slightly then the execution bars jump back into proper alignment.
I have the Linux version which I have run on both FreeBSD 12.2 and Ubuntu 20.04.1 LTS (focal) and seen the effect on both systems. I understand from aco-worker that this can happen on the Windows version.