Object tree filter should include a single filter for parts and lparts
The object tree filter sorts variables and parts into groups. Each filter can be for variables, parts, or lparts. Parts sorts loaded parts, lparts sorts unloaded parts. So if you want both loaded and unloaded parts to be placed in the same group you need to create 2 filters. It seems to me that this is what users would want to do 99% of the time, so either parts and lparts should be automatically handled together, or there should be a 4th option for 'parts and lparts' so that only one filter needs to be created.