Not sure if it's a bug but mine resets every time I reset my PC after I disable it from the menu bar.
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Tagging @wsmwk from the Thunderbird team to confirm if this option already exists.
On both Mac and Windows, the spaces bar stays closed for me across restarts.
So either your current state is not being saved in sessionstore.json on shutdown, sessionstore.json is being deleted, something is interfering with the state on startup. Perhaps an addon, or something like ccleaner.