After upgrade from Qt 5.12.8 to 5.14.1/5.14.2 occurs webview malfunction which looks like graphics artefacts(fig in attach). I do not remember if the Glitch started from Qt 5.14.0. As I have AMD (Mars) card onboard I have tested many graphic card configuration sets which did not helped. Actually one configuration, that which off's glamor X11 setup and turn's on exa accel mode helps. However this last one is really slow and unusable.
Glitch occurs only when using web browser Falcon and preview window in Kmail as also Akregator.
When using other IGP card which is Intel graphic card all seems to work OK.
My HW Configuration:
Laptop with AMD APU card, MARS generation.
My SW Configuration:
KDE Neon 5.18.5 with Qt 5.14.2.
When using older Qt 5.12.8 which is as a base in Kubuntu 20.04 problem do not exist and Kmail along with Falcon can freelly be used apps work correctly.