UAC dialogue box popping up at random while using sandboxed web browser

When using a sandboxed web browser in Sandboxie 5.30 and 5.31 beta on Windows 10 (happened in both 1809 and 1903), the user account control dialogue box will randomly pop up for no discernible reason. It seems to happen more often when watching videos, but it will still happen even if that's not what I'm doing. I've included screenshots of exactly what is being shown. This is on a clean, default box and the popups are coming on both Chromium- and Firefox-based browsers. Is there perhaps a setting I'm missing someplace?