I noticed a couple of issues while attempting to edit categories using mobile frontend on Android and Apple devices.
1. Navigated to https://en.wikipedia.beta.wmflabs.org/wiki/Dog
2. Tapped Categories at the bottom of the article
3. Added the Dog article to the Pets Category
4. Tapped Save
After adding the new category, the user does not see it in the list of categories until the user completely closes the categories page and returns back to categories again.
After selecting save there is an odd sequence of screens. Please see the video. Tapping close(x) should return the user to the article.
There is no way to delete the article from a category using Mobile Frontend.