I have several custom categories set up in Simplifi and in some cases there are a couple layers of nested categories. For example I have a top-level category called sinking funds. Under that I have subcategories for Home Improvement, Holidays, Vehicle Expenses, and Pets. Under Pets I have Pet Food, Grooming, Vet, and Boarding.
In my spending plan, I have a planned monthly amount for Sinking Funds as a whole. If I categorize expenses under one of the first-level subcategories (i.e. Home Improvement or Pets) then they show up under the planned spend as intended. But if I use one of the second-level subcategories (i.e. Vet or Grooming) then the expenses no longer show up in planned spend and instead are listed under other spend.