Other than using VLC instead of QuickTime, the other workaround for this issue is to convert the downloaded to pre-configured output format "Re-encoded MP4 (H264/AAC)" (or any other custom format where you force video re-encoding).
To request a download and convert action, pass the mouse over a detected video entry in the VDH panel, click the "..." sign that appears on the right, pick "Download & Convert".
To convert an already downloaded video, click the toolbar VDH icon to open the panel, pass the mouse over the "..." sign at the bottom left, pick the recycling icon with tooltip "Convert local video".