FYI - filed an issue for this. If not fixed for 1.9.0 should be fixed soon after https://github.com/NREL/OpenStudio/is...

Here's how to do it in the meantime in case anyone has an urgent matter:

• Create a boiler plant loop with the boiler on the supply side and the WaterHeater:Mixed (called tank) on the demand side
• Create a secondary loop, where we want to put the tank on the supply side. Let's call this DHW Plant Loop

Once your model is loaded

# Get storage tank
tank = model.getWaterHeaterMixedByName('Storage Tank').get

# Get Secondary loop
dhw_loop = model.getPlantLoopByName('DHW Plant Loop').get

# Add the tank on the supply side of the secondary loop
dhw_loop.addSupplyBranchForComponent(tank)

Then save your model, and when you reopen it the tank will be properly connected.

Edit

Here is a link to a measure on BCL that creates a WaterHeater:Mixed and sets the source and use sides.