We realised a different handling of the z-order for labels, dependent on the selected node.
On a rectangular node the label is painted entirely in over the node. Whereas when other nodes are selected, the labels are still in front, with exception of the border line of the very node, which supossed to be completely unterneath as well. See this strange behavior in the cutout below(e.g. DSG label).
We have no explanation for this and searching through the forum hasn't brought up any question concerning this issue.
Thanks for help in this matter.