I did switch to SCENE_ACTIVATION and it looked good, but when I switched back to CENTRAL_SCENE it also worked fine.
I do have a lot of Timeout errors on my zwave network, so it could be related.
Will sort out those errors first and re-test.
As long as CENTRAL_SCENE is used, a held-down event is received every ~300ms so a timer in the rule can be an easy workaround for detecting key-release as well.
Thanks for coming back to me on this.
I have started a new thread here.