In my former post, I was trying to hint at issues reported on the link I gave, someone there mentioned similar things. Sorry but I don’t use Gnome so frequently. Do you have gnome-tweaks installed and tried messing with the number of workspaces shown in the bar? - I gather these workspace-thumbnails generally are an add-on to the Gnome desktop, so the root-cause may be there, not T’bird.
Further reading through the comments on that page shows, the extension was developed for Gnome 30 something… apparently it does not work for newer versions yet.
It’s made for 3.34, as you can see. In GS 3.36, you just have to edit the Workspace Indicator official extension : search “vertical” and set it to false in the good function. Your layout will still be vertical but it will show the thumbnails in the panel instead of the number.
This is a very tiny fork of the original (GNOME official extension) Workspace Indicator, allowing to use it like when the Horizontal Workspaces extension is activated. The vertical workspaces layout is not modified; the workspaces layout is horizontal only in the top panel and the workspaces switcher in the overview is still showed. Please note that any global bug should be reported against the Workspace Indicator extension.