I believe the logic follows this path:
If the demand quantity forces the item to go below the triggered reorder point, the system will look to the replenishment source of that item’s warehouse detail setting.
If warehouse B’s replenishment source is set to Purchase, MRP will suggest a purchase of the item.
It warehouse B’s replenishment is set to Transfer, then it will suggest a transfer from the supplying warehouse.
We do this on a consistent basis in our company. What this forces you to have though is a single purchasing warehouse. Meaning, warehouse B could not send a signal to purchase the item from the vendor, it would suggest a transfer from warehouse A. If this Demand signal pulled the inventory of that item in warehouse A to fall below the reorder point for that item warehouse detail setting, it would trigger the replenishment source for that warehouse, which would presumably be the purchase source.
Hope this helps you with the logic that my company has experienced with this scenario.
Unfortunately in 23R2, this is no longer a viable path.