Outlook Mac send later option disappears with add-in

Question

We develop an add-in for Outlook 365 (using OfficeJS) which includes an on-send hook. Outlook for Mac has a send later function available by clicking a drop-down arrow attached to the send button, but as soon as our add-in is added, this send later function becomes unavailable (the drop-down arrow disappears). Is this a bug, or by Microsoft design? Is there any way to restore this functionality without removing our add-in?

David