ManageInventoryMethodId keeps setting back to 1

Hi,
We've got an interface between our Point of Sale and we've got it creating products in the NopCommerce database, all good. We also now create product variants, all good. Except we are setting the product's ManageInventoryMethodId flag to 2 meaning the product variant quantity is the quantity in stock that needs to be used. All good. For a while.

At some point, NopCommerce keeps resetting this back to 1 which we do not want. We need it to be set to 2, so the quantity in stock ties to the variant not the main product.

Why does this keep resetting?

Thanks,

Richard

PS we've had to write a thread which forces this flag back to 2 every minute, but obviously this is not an ideal solution