webenginequick introduces memory leak/crash/inaccurate video rendering during QML media player playing video

Simply adding QT += webenginequick without initializing or using any WebEngine-related API introduces buggy behaviors described in title. Two reproducers are attached. One is from original report. And the other one is simplified version. Comment/uncomment the QT += webeinginequick in .pro file to reproduce the issue.

However, I cannot 100% reproduce the issue reported by original reporter. Need more people doing more test. The behaviors in detail are as following:

On 6.3.1 :
The bug can be confirmed by both of us. webenginequick causes memory leak of VTDecoderXPCService. An abnormal memory usage can be monitored by activity monitor. The normal memory usage should be around 180MB and kept that way without webenginequick. However, after linking to webenginequick causes memory usage to rocket to ~800MB and keep increasing until OS is running out of memory.

On 6.4.2:
The original reporter says that video can be 'played normally', judging from logging. But no video rendered whatsoever, i.e., the application window is totally blank. I cannot reproduce the issue, that application simply crashes due to SIGILL.

On 6.5.0:
The original reporter states the same as for 6.4.2, i.e., video being played but not rendered. I cannot reproduce it, that video is just played and rendered normally.