Somehow, KDE Connect treats a media stream happening on a connected device the same as if it's playing on your local device. If you're playing a video on your laptop in Firefox it will add one of those "music player" things in your phone's notification shade, allowing you to control the video from your phone.

Android automagically pauses everything it deems to be "media playback" until the end of your call, thus also pausing that Firefox video on your laptop.