tiled window cannot be resized on mouse drag

Hi there, an odd bug under kubuntu 20.04, I can't resize tiled windows which touch my top panel.
I set my workspace like ubuntu, a panel on the left holding the apps' shortcuts and a top panel with the system tray and the global menu. I use quarter tiling.

So long as the tiled window title-bar touches the top panel, I can't resize my window horizontally. However, vertically, it works fine. To be accurate, horizontal resize only works towards the outer edge: try to picture a window tiled to the top-right corner or to the right half of the screen. Dragging the window left border towards the left end of the screen works, but not dragging it to the right side of the screen to reduce the window size. Same behaviour when I enable window snapping to resize 2 windows together. However, when windows are tiled in the bottom corners, no problem whatsoever.

I didn't experience this bug in previous versions of kubuntu. Any idea where to look to resolve this issue?
By the way, holding Alt+left click works fine to resize all windows.

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: kwin (not installed)
ProcVersionSignature: Ubuntu 5.4.0-24.28-generic 5.4.30
Uname: Linux 5.4.0-24-generic x86_64
NonfreeKernelModules: nvidia_modeset nvidia
ApportVersion: 2.20.11-0ubuntu27
Architecture: amd64
CasperMD5CheckResult: skip
CurrentDesktop: KDE
Date: Fri Apr 17 09:56:06 2020
SourcePackage: kwin
UpgradeStatus: No upgrade log present (probably fresh install)