Instant deposits are long time gone because majority are really requiring at least 1 confirmation in the network before the deposit would reflect out into your balance
and with regards with double spending issue then this is the main reason on why they do changed up routes or set up to be like this.
Or the reason could be different, so we can speculate on things until the actual response arrives from the security team why the account was put on hold. This double-spending issues also can be a reason for that but it's less likely to be in particular.