I have asked around and saw forums in hardware zone, that its normal by OCBC to behave this way or if they suspect or not happy with you, they will “review” you. Then they can just demand to cease services with you. But as they close the account they have to report to the relevant authorities.. so in the future... even opening with another bank would be difficult.
They are exercising their legal right to stop doing business with you for any reason.

In your case, I suspect it's to avoid any risk of being implicated in a police case. Business as usual for them. They've ended relationships for less, including just being unprofitable.
To address your point on whether it'll be difficult to open accounts with other banks: no, if they terminate the relationship on their end, they don't have to report to "the relevant authorities".
They can report suspicious transactions, but I believe other banks don't have access to these reports, only the police: https://www.police.gov.sg/Advisories/Crime/Commercial-Crimes/Suspicious-Transaction-Reporting-Office
I've been posting this for a while: only stupid people continue to bank with OCBC.