Slots with staff availability not working ok

[Valentina R]

To reproduce: Create an appointment with Inventory 1 and no availability rules. Duration is 90min and interval is 30min. Assign 2 staff members, one with availability Sunday 11AM - 12:30PM; the other with availability Sunday 11:30AM - 01PM. Add the 11AM slot to cart and the day is clickable but has no slots to choose from.

Reported here: https://bookingwp.com/forums/topic/slot-appears-unavailable-if-overlapping-appointment-with-another-provider/

[Željan Topić]

Fixed with 4.0.1