There is slightly newer firmware but not sure if they have done anything to fix that. I believe another firmware update is coming very soon as well.
https://www.support.getzooz.com/kb/article/300-zen21-on-off-switch-change-log/
I used to see this issue sometimes on my ZEN27 switches which were the first I installed (I have 21s also). Not sure if it was firmware updates, hub updates, or my custom driver but I have not seen it happen in a long time. I would make sure your device and hub firmware is up to date. You could also try my driver to see if that makes a difference. I know Bryan's driver is no longer being maintained since he now works for Hubitat. [RELEASE] Zooz ZEN Switches Advanced Drivers
