You have the option to remove the video play-bar on your videos.
Videos uploaded to your Thinkific site are hosted with Wistia. Unfortunately, due to limitations of iOS, when a video is viewed in fullscreen, mobile devices do not allow you to use the Wistia video player and instead use their own native native mobile player which will display a play-bar.
Wistia also has more details on this functionality here: Inline Playback for mobile roll out
We understand that this is not ideal in some cases, and if you have any feedback on this, please don't hesitate to reach out.