I've had to go through a little bit of dance with Ubuntu. The fact that Qt acknowledges the mistake and has patched it (and now merged the patch) doesn't seem to be enough reason for them to patch their own qt5.12.8 webengine packages.

The maintainer asked for more details and possibly a test case. He said the acknowledged upstream mistake would be enough for him, but not for whatever review process Ubuntu has in place.
I've obliged with tons of detail and a small Qt test-case project that demonstrates the issue. Hopefully that will be enough!