Looking at the API documenting for channel rewards, I found the following:

https://dev.twitch.tv/docs/api/reference#update-custom-reward

The Custom Reward specified by id must have been created by the client_id attached to the access token.

I'm not really sure what the design was behind locking channel point updating to only the client that created the channel point, but it adds a lot of unnecessary work on 3rd party integrations to work around this. 3rd party apps that want to allow users to customize the channel points they already made now have to go through the hassle of re-creating them through the 3rd party app just to be able to use.

Although I can see some value to the idea of placing a restriction on what rewards a client can update, there should be some more control given to the user to let them work around this. Either a setting on individual rewards that says "Allow 3rd Party Applications To Edit" or just a blanket ability to edit rewards created directly through Twitch's UI (IE, don't allow Client A to edit Client B's rewards).