The "dynamic volume % display on Gizmo"?
While in many apps I get a slider popup when I change volume (using the physical volume buttons on the phone), that is an Android feature, and it doesn't happen in Gizmo. All I get in Gizmo is a popup that states the volume percentage, and that only stays on the screen for a second or so, and doesn't pop up again if I immediately make more volume changes. It only pops up again if I stop making volume changes, wait a second or so later, then make another change.
I'm not sure that is the way it always worked, but if it has changed I would suspect Android changes. I think the audio system on Android has been overhauled, but I'm no expert on that. Maybe the way it handles older Android Apps has changed.
I have Gizmo 21.0.1. Yours that same?
I still have full control playing to my phone or a MC Server, including using Gizmo as a Remote Control in Theatre View. I can connect to and navigate my server, displaying Album, Artists, etc.
Does Gizmo connect for you, and are you able to navigate audio files? If not, try deleting the record of the server (select Mode, press and hold the server record, select Delete from the pop up menu), then re-add that server using its Access Key.
Does that fix anything?
If not, is your LAN still marked as a Private Network in Windows? If not some network traffic between your phone and MC Server may be getting blocked.
Has anything changed in your Firewall settings. Windows Updates will change some things.
Still stuck? Perhaps Clear the Gizmo Data and Cache in Android. That is under Apps > Storage. Google how to do that if you don't know.
Still not working after you set it up again? Uninstall Gizmo, reboot your phone, and reinstall Gizmo.