Looking at the image, I would say this is related to the specific function and is due to the fact that it is not among the supported ones
. I logged the implementation of this in our backlog and you can subscribe to the item at Implement SUBTOTAL function
Meanwhile, you could try creating a custom function by following the approach explained in this help topic
. There's also a live demo of this in our WPF developer-focused examples
I hope this helps.