Deploy to existing RG

I am trying to deploy to an existing RG, the existing VNet has 4 subnets. My template for the new VM to be deployed only needs to reference 3 of those subnets. When I try to deploy it I get an error that the 4th subnet was trying to be deleted but is in use. Why would the subnet not referenced be deleted?

Start by reviewing the deployment template and the existing VNet and subnet references described in the issue. Reproduce the deployment against an existing resource group, then determine why the unreferenced subnet is treated as removable; done means documenting the cause and a validated deployment behavior or fix.

Written by the indexing model from the issue text.