When I open mediaframe configuration dialog, Fill mode is always set to "Preserve aspect fit". If I change it to something else, I cannot apply the change. Reproducible: Always
I can confirm this on Arch Linux and Plasma 5.7.4.
Same bug in plasma 5.12.1. When I select another "fill mode", "Apply" button is not enabled and "ok" button does not apply the new setting.
Fixed in Git master; not fixed in Plasma 5.12.4.