Sound Indicator disappears on vertical MATE panel

If you use the sound indicator with a vertical panel, it will disappear when an app like Rhythmbox that adds itself to the indicator's menu is opened (or closed).

You can see this by adjusting the properties of the default Ubuntu MATE top panel by changing the orientation to left. The sound indicator is there, but if you open Rhythmbox it disappears. If you change the panel back to top orientation, the indicator shows up again. (If you then change it back to left, the sound indicator does show up, but will disappear again if you quit Rhythmbox.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: indicator-sound 12.10.2+17.10.20170829.1-0ubuntu1
ProcVersionSignature: Ubuntu 4.15.0-10.11-generic 4.15.3
Uname: Linux 4.15.0-10-generic x86_64
NonfreeKernelModules: wl
ActionStates: ({'mute': (true, signature '', [<false>]), 'phone-settings': (true, '', []), 'mic-volume': (true, '', [<0.31622314453125>]), 'scroll': (true, 'i', []), 'high-volume': (true, '', [<false>]), 'desktop-settings': (true, '', []), 'volume-sync': (true, 't', [<uint64 0>]), 'rhythmbox.desktop': (true, '', [<{'running': <false>, 'state': <'Paused'>}>]), 'rhythmbox.desktop.greeter': (true, '', [<{'running': <false>, 'state': <'Paused'>}>]), 'volume': (true, 'i', [<0.24173394559660655>]), 'previous.rhythmbox.desktop': (true, '', []), 'indicator-shown': (true, '', [<false>]), 'root': (true, '', [<{'title': <'Sound'>, 'accessible-desc': <'Volume (36%)'>, 'icon': <('themed', <['audio-volume-medium-panel', 'audio-volume-medium', 'audio-volume', 'audio']>)>, 'visible': <true>}>]), 'silent-mode': (true, '', [<false>]), 'play.rhythmbox.desktop': (true, '', [<'Paused'>]), 'next.rhythmbox.desktop': (true, '', []), 'play-playlist.rhythmbox.desktop': (true, 's', [])},)
ApportVersion: 2.20.8-0ubuntu10
Architecture: amd64
CurrentDesktop: MATE
Date: Thu Mar 1 10:12:40 2018
InstallationDate: Installed on 2018-03-01 (0 days ago)
InstallationMedia: Ubuntu-MATE 18.04 LTS "Bionic Beaver" - Alpha amd64 (20180301)
SourcePackage: indicator-sound
UpgradeStatus: No upgrade log present (probably fresh install)