I have the same issue. Rebooting both the phone and the watch did not fix it.
Also, in my case, all the widgets positioned after the customized widget will not show on the watch even if they are populated within the app. Eg if I put the custom widget in the first slot, none of the other widgets will be displayed on the watch.
Lastly, if I copy the customized widget in multiple slots, it is the same in all the slots on the phone, despite any customization applied in the apps.
Any ideas? Thanks!