MBP resolution changes when connected to an external display Until recently, I was able to connect an external display to my 2013 MBP Retina without a problem. As of the past week or so, connecting an external display results in the resolution of the MBP changing from 2880x1800 to 1440x900. The external resolution is unaffected. I'm unaware of any particular system changes prior to when this problem started.

I use Display Menu to manage the resolutions of both displays and this has worked well for me for the past year or so.

The problem sounds like it's the same as described here: Is there a setting to make the Mac not change resolution when connecting to an external display? except that I'm not trying to mirror the MBP display and mirroring is not switched on.
I guess you have selected Scaled resolution with "More space" option for your MBP screen. 
Have a look at that once you connect the second screen. For me connecting external display switches this option to "Default" for my MBP screen automatically.