Some mobile devices use the device's default player when you select fullscreen. In other words, when viewers watch a video on mobile and select fullscreen, they will be taken out of the Vimeo player and into the device's player, which removes the interactivity features from the video.
For example, when you go fullscreen on an iOS device, you are using Quicktime, which doesn't support interactivity features.
For this reason, we recommend embedding your interactive content to the width of the screen to make the most of the available real estate on smaller screens, and hiding the fullscreen button from viewers.