Custom Carrier Service - Multiple Locations

Shopify Partner
18 0 2

Has anyone seen strange issues with using a custom carrier service + multiple warehouse locations?

I assume if you have 2 locations set-up, location A being primary, that all orders if in stock in location A, will make a single call to the custom carrier service with location A as the origin.

And if out of stock in location A but in stock in location B, that it will make a call to custom carrier service with location B as the origin.

Is this the correct expectation?

For some reason, with a product in stock in both locations I seem to see 2 calls to the custom carrier service - 1 for location A as origin, 1 for location B as origin, and the only rates it ever displays are the ones returned from location B.