Craft undocks from target Space Station when approaching within 1 km

I have tried this four times by now.

Steps to reproduce:
- Start the Sandbox with the 'Base' craft
- From Map View, select DSI Space Station in the list and select it as target
- Transfer fuel from the lower stage ( section under the solar panels) to the upper stage.
- Upon fuel transfer completion, proceed with next stage twice (1: stage separation, 2:
- Approach DSI Base Station
- When the proximity between Base and DSI Space Station < 1km, a undocking sound is heard. The undocking craft is a craft from the target.

I managed to get around this, by quickly saving before the 1 km mark was hit and then going back to that. From then it was no issue (at least, I did not hear the undocking sound). However, launching this from the sandbox and doing it as I told in the reproduction will undock a craft from the Space Station.