Monzo don’t block specific numbers:
Tipping off legislation means they legally can’t tell you anything, meaning that the lack of response until any investigation has been completed isn’t necessarily their fault.
The lawyer is guessing and likely has little more insight into Monzo’s thinking than any of us on the forum, though I grant they’ll have more visibility of your circumstances. I’m not aware of there being dramatic changes in Monzo’s ‘Know Your Customer’ checks that would lead to compliance gaps.
Only Monzo could give you that information, anyone else - us here or the lawyer you spoke to - would only be guessing. I’m sorry to hear your account was closed, and regret that the law leads to it unlikely ever being clear why these decisions are made.