This happens for me with class nodes that I put in any diagrams. I have a similar issue with the UML case oval node. I can edit the label by double-clicking it and typing, but if I edit the label via the Text part of properties, then the changes are discarded every time.
Similarly I can edit the name of a class node by double-clicking, but I can't edit it by changing in the properties, any edits I make there are discarded.