Getting Yellow Nodes by clicking on 'Keep subprogram file updated with this program'

Hello everyone, I am currently facing a problem with subprogram.

When I click on ‘Keep Subprogram File Updated With this Program’ and then save and close the main program. And again when I open the main program. It shows yellow nodes with that subprogram.

I have not been able to understand why is this happening. Please help me out. thanks!