Actually it is not bug, this is how our system supposed to work. Booking Item has no connection with the booking slots
Booking slots are reserved based on the product assigned to a booking item.
If you have multiple products assigned to a booking item, each product will have its own slots.

Anyway, there is a work around for your requirement,

You can utilize the Resources  Feature ( Which is available only in pro version).
Resources  feature can use to add common resources needed for booking, it can be additional items served,  cleaning cost, or any other additional fee can be collected using this feature.

For your requirement, please go to Resource section , and create a Resource, with any name , 
and check the following options .
ensure you have un check "Is Optional", and you can hide showing this resource for customer by Ticking "Hide from Customer"

And the important thing that you have to check this is a limited resource option at the bottom 

 Once you created the resource, you have to assign it to the Booking Item .