Workspaces not working in Slack

[notklaatu]

Slack version: 5.10.0
OS: Linux (Slackware 15.0)
Install: Flatpak

What's happening

Trying to sign in to one of my employer's two Workspaces. In Firefox, this works as expected: I search for a Workspace, click on View Details, and click Join and Open.

But in Franz, when I click on View Details I get a button to Join and Open in Slack (which launches Firefox).

What I've tried

I've tried joining the Workspace in Slack's web app in Firefox and then opening Franz, but Franz still wants me to choose a Workspace.