Volume slider fails to appear on Plasma when hovering over start menu icon

When an application is playing audio, one can hover over the plasmashell app icon to see the preview + volume slider.

This volume slider does not appear for me when trying to run Steam over Flatpak, but native Steam shows it just fine. Other Flatpak apps like Haruna also work fine. Tested by playing a video in a Steam store page.

This is the slider in question:


Random store page with a video to play: https://store.steampowered.com/app/230410/Warframe/

Previous report for Plasma here - https://bugs.kde.org/show_bug.cgi?id=492454

Reproduce the issue by playing the Steam store video at the linked Warframe page through the Flatpak and native Steam versions, then hover over the plasmashell app icon. Compare the preview and volume-slider behavior; done means the Flatpak Steam case matches the working native Steam behavior.

Written by the indexing model from the issue text.