How does your code go from the repo to the users? How do you deal with connection draining? Do you make sure that requests using the old code aren't interrupted? Do you spin servers up and down based on load? Do you have websocket connections that can last for hours?
Many times as a mobile developer I have to work on apps without the API ready that was crucial for the feature I was implementing. Either the backend was developed by another team that was not entirely in sync with us or our backend team had no chance to implement those endpoints earlier. For this reason, I was not able to satisfy the Definition of Done but it does not mean that I have implemented the UI only.