UseSlidingInvisibilityTimeout not working sometimes

[strigefleur]

Having configuration

InvisibilityTimeout = TimeSpan.FromMinutes(5),
UseSlidingInvisibilityTimeout = true,

I'd expect it to let long running jobs to execute without setting huge InvisibilityTimeout.

However under some circumstances (alas, can't reproduce intentionally) - like for a week it is fine and than just no - the task is interrupted after 5 minutes (InvisibilityTimeout I guess) with OperationCanceledException and then is tried to requeue (again sometimes there's also a warning about another worker that fetches the task).

Any ideas?