The Esus Recon units' mask button shows slightly erratic behaviour. Mask will sometimes be grayed out after shift-selecting several units (even when all of them are eligible to mask this turn). If you then select a single unit it will still be grayed out. However, if you force-ungroup them (button with the split-arrows) it will then work again.
I'm on version 8.10
I'm on version 8.10