The junior programmer might just want to be heard or is having a hard time understanding your code or previous code.
I guess they have been working on that repo longer than you.
The development might be fast and PR quick if you asked their opinion before starting the implementation.
That way as senior you will be including the junior in the implementation ...
You have two different problems here.
One is: You make a change. Reviewer reviews it. Reviewer spots code that may or may not be objectionable near your change, and comments on it, possibly requesting a change.
The answer to that is an absolutely steely reply that this code has nothing to do with the change you made, is not something that he should ...