For tablet and phone:
- load any article
- scroll down a bit in the article until the top navigation bar goes away
- tap the TOC icon and make a selection from a different section than the one you are currently seeing
- ensure the top navigation bar has re-appeared after TOC selection is made
Testing on iPhone 6 Plus (iOS 11.1.2) and Wikipedia app 5.7.2 (1267). According to the attached video making a ToC selection still doesn't make the navigation bar re-appear (however, this may not be quite what is wanted, it does reappear if you scroll up a bit in the article afterwards).