I just tested it out myself. It looks like the event is not propagating down past the YAMS plugin. Please try changing the event order to put the new plugin before YAMS for OnBeforeDocFormSave. Then it works.
I can reproduce the behaviour you have just described, but without further investigation I can’t be sure why it’s not working on first save. The OnBeforeDocFormSave event should get executed when a new document is saved and not just when an existing document is saved so I thought this would just work.