We use a spine template that lets us import in a 1x1 mask and a mask force that allows us to resize the bone and set a specific output size. We simply import it as a skeleton and drag and drop the bone into our main skeleton and resize the slot. Since v3.8.13beta this process will not work. If you drag in the bone, the mask is broken and can no longer be resized. In order to get around this we now must create a bone in the main skeleton and import the slot instead. It's not a big issue but it's weird and concerning of what other things might be broken. We also tried importing in to the skeleton, which also does not work and creates a duplicate of the slots children.
Here's a copy of the file we use. simply follow the instructions to recreate the issue. Then to see how it should work, create a bone in the main comp you're working in, then drag the mask and the mask force slot into that bone.
As a side note, this is a REALLY useful file if you need to make a pixel perfect mask or generate files at a specific size (it both expands and contracts the area to a set value). In order to make a perfect square mask you have to write it in json and import it, it can't be done with the editor. Maybe this can be a feature that is added?