A quick search of the QtBug database reveals the following *potential* cause:
https://bugreports.qt.io/browse/QTBUG-134610
I do not know the calibre codebase well enough to know if multiple signals are being temporarily disconnected from an object or not. But this is one of the only still open issues associated with the QAccessibleCache in QtBugs.
There seems to be a Qt patch for Qt 6.8.4 that detects multiple signal disconnections that removes the "destroyed" signal that the QAccessibleCache code needs to stay in sync, and to warn people not to do that.
Of course, it could just be a new Qt bug as well.