One way to solve this pretty easily is to click on your Layered Blend Per Bone node in the parent/template Anim BP and create a OnInitialUpdate function binding in the Details menu.
Keep the default implementation the parent as it is.
Then in your child Anim BP, you can override this OnInitialUpdate function in the Functions → Override dropdown and do the following:
And specify the name of a blend mask that exists on your skeleton. The call to the parent doesn’t really do anything here, but it’s here for completeness.
You could also instead choose to implement this function in the template/parent’s OnInitialUpdateas a default implementation. Then you could also make the Blend Mask Name string a variable so all you’d need to do in the child is paste in a different blend mask name in the variable if you needed to choose a different blend mask per child.
Hopefully this helps anyone who comes across this issue.