I don’t see any contact details either. However, based on their blog, it seems that Moxie Marlinspike (the founder of Signal messenger) is behind it. So it might be worth trying to reach out to him directly via Instagram or X.
I was just about to try that, but unfortunately it looks like Posteo only offers paid accounts.
I just created a new thread. Please let me know if anything is missing or if I should report the bug elsewhere.