It’s telling me to join an alliance even though I’m already in an alliance
I'm seeing this too, only on the Remastered event version (Demastered shows my alliance as normal). In case it's relevant I'm playing on Steam and I joined the event fairly late after a couple days away.
Edit to add the main node screen also oddly shows my score should make me 18/20 in my alliance, it's just the Event Rewards screen that doesn't seem to acknowledge my alliance: