I noted a strange behaviour on the bots: The waited in front of a roboport and did nothing. Complete freeze. They carried a maintenance part. I had some provider chests, but no storage chest. So they were not able to carry the item to the port as the port does not seem to allow these items.
As soon as i placed a storage chest, everything worked again. The bots delivered the parts to this chest and flew home to the port.

For what i know, this is the default behavior of construction bots. The same would happen if you mark some entities for deconstruction: Without a storage chest, the bots will hover over the place where the deconstructed entity has been, with the mined entity still in their inventory. I think it's generally recommended to have at least one storage chest available within each network dedicated to construction, repair or - in this case - maintenance.
Because this is vanilla behavior, i'll regard this issue as "not a bug".