I'm not sure if it's a QT issue, but there is a possibility:
I have a PyQT5 "digital picture frame" program running on RPi Zero W. I have used it for almost two years with Raspbian Jessie, but now I have updated the system to Raspbian Buster and compiled QT 5.12.4 and PyQT 5.13. The program works fine for several hours, but after that a kernel error appears in the log and the GUI freezes. Every "background tasks" (bluetooth, mqtt, etc.) countinous to work fine.
More information: [Raspberry Github Issue|https://github.com/raspberrypi/firmware/issues/1251]