MATE Focal "amplified" sound controls do not sync with keyboard buttons

I probably have the package wrong but I was grabbing at straws :^)

Just recently testing Ubuntu MATE 20.04.1 release candidates and I notice that you can increase volume beyond 100% by opening Sound Settings, and once you've done that you can also increase beyond 100% using the volume "widget" in indicator-applet, but I've tested on three different sets of hardware and the volume buttons on the keyboards fail to increase beyond 100%.

In fact if you're above 100% and try to bump the volume down slightly it will drop all the way to 100% and can't be increased beyond that without access to the "volume widget" so if you're viewing videos in full screen you first have to exit full screen and then readjust above 100%. So it's kind of a show-stopper for those using the OS on an HTPC.

This had effected Ubuntu Bionic but was fixed in 18.10 and remains fixed in Focal - see bug #1817625. I've no idea what they changed to achieve that. Testing Focal live sessions only it seems that this same Focal bug also effects Kubuntu and Xubuntu, but they were only tested in live sessions whereas I've tested Ubuntu MATE as an installed OS.

As mentioned above in Ubuntu this now does work poroperly, and also works properly using the flashback session but you must either first set "above 100%" in the Ubuntu interface or go to dconf-editor > com.ubuntu.sound, which I did also try in MATE but to no avail.

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: libmatemixer0 1.24.0-1
ProcVersionSignature: Ubuntu 5.4.0-42.46-generic 5.4.44
Uname: Linux 5.4.0-42-generic x86_64
ApportVersion: 2.20.11-0ubuntu27.4
Architecture: amd64
CasperMD5CheckResult: skip
CurrentDesktop: MATE
Date: Sun Aug 2 17:45:16 2020
InstallationDate: Installed on 2020-07-31 (2 days ago)
InstallationMedia: Ubuntu-MATE 20.04.1 LTS "Focal Fossa" - Release amd64 (20200730)
SourcePackage: libmatemixer
UpgradeStatus: No upgrade log present (probably fresh install)