No idea. I've seen it behave this way for at least the past year. I just tried it on a different Mac and it behaves similarly, except this other Mac causes QT t

No idea. I've seen it behave this way for at least the past year. I just tried it on a different Mac and it behaves similarly, except this other Mac causes QT to open up the File Open dialog immediately on launch. (Version 10.5 of QuickTime player for me too. macOS 10.15.7 though.)