> "we refuse to tell you why" and in some cases holding onto their money for months

I don't know anything about Monzo specifically, but perhaps anti-money-laundering laws might be the reason. Monzo is presumably obligated to take steps against money laundering, under UK law, where it's essentially an offence to say Our system has flagged you as a potential money-launderer. [0] Freezing your account without comment may be their safest course of action.

edit I see this explanation was mentioned in the reddit thread.

[0] https://www.lawsociety.org.uk/en/topics/anti-money-launderin...