Discovered a serious bug in QML

Environment: Windows 10+Msys2+Qt5.15.10-5.15.16 (MinGW).
Please refer to the attachment for the code and debugging.
Trigger situation: Appears after a fixed number of runs.
Explanation: This issue occurs in Qt5.15.11-5.15.16, but was not found in the version 5.15.2(official) and 5.15.10(MSYS2).
Code：
Output：

tpromise is a Promise object in JavaScript, while game, d, and $pymo are all regular JavaScript objects.

Actually, after my step-by-step debugging, I found that the code(game.d.$pymo.wait_promise = tpromise placed the tpromise object in game.d.$pymo.background, which is a serious error. If I run 'game.d.$pymo.wait_promise = tpromise;' again, the game.d.$pymo.wait_promise value will be correct.