In the default setting, the app uses preview images for the preview playback, in order to keep the audio and video playback in sync. Particularly on devices with slower hardware or when using movie filters.
Depending on the device speed and movie effect selected, some frames may be dropped when played in preview mode. These frame drops happen automatically, otherwise, your audio would no longer be in sync.
Changing the preview quality to a lower preview quality setting may result in less frequent frame drops.
Rest assured, when exporting the video, the app will use high-quality images only, regardless of the preview settings.