This is not expected behavior. If your utilization is set to 1 call and 2 chats, the agent should receive up to 2 chat interactions even when they are on the phone.
Could you please open up a ticket with support to report this issue? They would likely need the agent information (name, id), the time as well as possibly the call interaction the agent was on. The more details you can provide the easier, but send us what you have.