All Qt 6.2.0/6.2.1 QML apps appear with a white background on my Google Pixel 4a phone running Android version 11.
This only seems to be with Android targetSdkVersion 30 defined in **build.gradle.
It appears to be only with Android SDK 30 as SDK 29 seems to run fine. It is also only using arm64-v8a ABI. When I tried armeabi-v7a it appears correctly.
I also tried Android build SDK of 30 and 31 and both show the problem.
Sorry but so far I have only managed to reproduce this on my Pixel 4a with the arm64-v8a build but it could be an issue on other phones.
I have attached a small test program as an example of the problem.
If you want me to try other configurations with my phone then let me know.