The "display only selected part" Checkbox in the model editor, doesnt show up when clicked. It shows up only after clicking it and then pressing the arrow to select the next part.

Edit: seems like it happens only the first time. After has been done once it is working properly afterwards.

you must have at least one part in the mesh

I do have multiple parts, otherwise I could not press the arrow to switch to the next part.

It happens only the first time after editor startup, you can repro it yourself:
- Start the model editor
- Select or import a model with multiple parts
- Click the checkbox, it doesnt show up as checked (but it is checked)
- Click the arrow to select the next part, now the checkbox shows up

Yes because there was no selected part at the start