From the Reserve Strap blog:

In recent WatchOS releases, Apple made the decision to remove all functionality from Apple Watch’s accessory port thereby blocking Reserve Strap’s ability to charge the Apple Watch. This was a deviation from how the port functioned in all previous WatchOS releases and appears to have been a deliberate effort to block development of third party smartbands.

This action was surprising given that prior to this WatchOS update, Apple had been an advocate of our product–going as far as inviting us to Cupertino to show them early prototypes as well as placing pre-orders for many Reserve Straps. Additionally, Apple continues to run old versions of WatchOS on their in-store kiosks in order to utilize the functionality of the accessory port.

Wow.