Transparency doesn't last more than few seconds

Compositor is on (or at least it is ticked), but no transparency, when I change the rendering backend to XRender, the panel becomes transparent for about 4 seconds, then get back to non-transparent. both OpenGL version don't activate transparency at all.

ProblemType: Bug
DistroRelease: Ubuntu 15.10
Package: kwin 4:5.4.2-0ubuntu1
ProcVersionSignature: Ubuntu 4.2.0-16.19-generic 4.2.3
Uname: Linux 4.2.0-16-generic x86_64
ApportVersion: 2.19.1-0ubuntu2
Architecture: amd64
CurrentDesktop: KDE
Date: Thu Oct 15 17:35:45 2015
InstallationDate: Installed on 2015-10-13 (1 days ago)
InstallationMedia: Kubuntu 15.10 "Wily Werewolf" - Alpha amd64 (20150924)
PackageArchitecture: all
SourcePackage: kwin
UpgradeStatus: No upgrade log present (probably fresh install)