Linux: QSaveFile creates files with 000 permission on selected 64bit systems

I have two rather similar Arch Linux setups: One on a physical laptop, another one in a VirtualBox VM on a Windows 10 host. Only in the latter installation, files that are newly created by Qt applications get permission 000. This includes files created with, e.g., kwrite, but also files that are put in ~/.cache by other applications. As far as I can tell, non-Qt applications are not affected.

This might be related to QTBUG-69436; however I use glibc 2.28 and a 64bit system. Unfortunately, I was not yet able to check it the patch from QTBUG-69436 also fixes my problems.

Strangely (incidentially?), this behavior occurs since my system partition (in the VM) got full during an update: Of course I cleaned up some space after that and rolled back to a clean btrfs snapshot and successfully updated from that state. I also cleaned ~/.cache. Starting KDE only worked after repeatedly manually fixing permissions for newly created files in ~/.cache after several unsuccessful attempts to start KDE.

I attached two outputs of strace for the program given in QTBUG-69436; one from the VM and one from the Laptop (where everything seems to work fine).