The key from the Tor Browser developers involves a long-term identity, and a set of shorter-term signing subkeys. These subkeys last for a year or so, and they make new ones periodically, because "key rotation" is a good way to limit the scope of damage if something goes wrong sometime.
So, you likely have the old subkeys since you "saved the tor browser developers signing key on your computer", but if you fetch a new copy of the key, you'll get the new subkey too. Hopefully everything will work smoothly from there.
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