netbox: Device_count is not updated on device roles if a device is removed/add to the role

Environment

• Python version: v3.6.8
• NetBox version: v2.6.5

Steps to Reproduce

1. Create a new device role
2. Add an existing device to it
3. Check the api response on device_count
4. Remove the device from it
5. Check the api response on device_count

Expected Behavior

I expect the device_count to be correctly updated

Observed Behavior

When adding an existing device to the role, the role’s device_count stays at null. When a new device is created and added to the role directly, the device_count is still null.