Despair, I just tried now (07:29 AM UTC) markets closed, and it works fine, either for single leg or combo whatif orders.
See attached screenshot, one is for single legs, one is for combo. I use WHATIF orders to get the margin requirements before I transmit the order.
Did you track error messages? There can be
errors code 110: 'The price does not conform to the minimum price variation for this contract'.
I remember when coding my app that this error forced me to manage 'minimum price increment' through marketRuleId, for normal orders AND whatif orders.
Francois G