There were a few changes made here in the supplier section due to queries raised elsewhere (see here for example), so it may be the case that this has affected it, although I can’t say for certain - I’d have to consult with a developer to verify this.
Adding 23% to the VAT box I mentioned above should do the trick however. If you create a new invoice, the 23% should automatically appear, but if you’re editing an invoice (which I assume you as you mention January’s invoice), it will only appear if it’s enabled in your account.
Did you originally create February’s invoice with a different rate and then update it to 23%, or was it created with 23% in the first place?