Logistic robots occasionally go out of the map and the jobs are locked by them.
I've checked the source of this mod, and it may internal bug of Factorio.
By disabling this mod, those robots started to come back to base and it does not happen anymore.

Very strange thing. I've been running my save on a dedicated server for hundreds of hours and did not notice this. I'm not sure how they coded the interactions with roboports under the hood, but this mod does not change it explicitly, so I cannot do anything about it, unfortunately.