it looks a sim issue because when you try the same with the default/stock navdata of MSFS2020, you will see exactly the same behavior. As you have figured out, when you select the STAR only (approach is automatically), all looks good but when you add the approach (ie. ILS28) then the STAR will be cuted for some reason.
STAR BAGS2L only:
Also what is strange - the waypoints will be visible of the STAR.
Now, I have added the approach ILS28:
You see, the STAR waypoints from before will not be deleted as normal. So, something happens in the background, when ASOBO connect the STAR with the approach. Again, this happens also with the default data and also, if happens in all FMC/S, Garmins … possible a general logical issue which isn´t so easy to fix.
Sorry, but after checking the procedure, I can´t find any issue.
Is is possible, that your friend has select the STAR only? When not, is it possible, that you can ask him, which AIRAC and which revision he is using?
Thank you very much,