[REG 5.15] OpenGL rendering issue / flickering on scrolling and animations

Hello,

i have a cross-compiled armhf embedded build of QtWebEngine 5.15 (with V8-snapshot support).
Now i have the issue that the rendering using OpenGL/EGLFS has some flickering on every frame rendered, e.g. during scrolling or animations). (The issue is gone with the "--disable-gpu", but that obviously also eliminates WebGL support).
The issue seems like some sort of flickering or like the previous frame is rendered before the new one. Also on the very first rendering of the WebView there are some artifacts shown from a past render state.

Hope it can be seen on that video: https://www.youtube.com/watch?v=yBMDasx3zUA

The embedded device is using some mali-blobs for OpenGLESv2 support. While this exact same setup works fine and runs smoothly with Qt 5.12, Qt 5.15 showed up this issue.

I also tried with the following flags, but the issue is the same:

./quicknanobrowser https://get.webgl.org/ --no-sandbox --ignore-gpu-blacklist --egl --disable-es3-gl-context --use-gl=egl --enable-checker-imaging --num-raster-threads=3 --enable-gpu-rasterization --enable-native-gpu-memory-buffers

Edit: added buildconfig if needed (built with changeset from QTBUG-84720, until 5.15.1 is released)