QUICKTIME player does not show the picture when stretching an image... I have well video if i don't click on stretching. When there was a Sierra, there was no such problem
You may want to review THIS DISCUSSION. They seem to be having the same/similar problems on their 2017 MacBook Pro devices. However some users (with dual graphics cards) report the issue disappears when the alternate card is selected.
Users with older MacBook Pro systems have also reported their systems auto-switch from "Fill" Full-Screen mode to "Panoramic" Full-Screen mode when the controls/cursor disappear but the problem goes away when system is booted in the "Safe" ("Shift" key) mode. Was wondering if your issue also disappears in "safe" mode.