Locked objects automatically unlocking after leaving isolation mode / mirror

Hi All, 

Not sure if a bug or intended setting, but annoying & can't find any details online from people w/ similar situation. Essentially I'm using an image as a background reference to trace and I'll set it to back and lock the layer so I can't interact with it. As I'm drawing with the Pen and drawing shapes, I'll utilise isolation modes and most specifically, 'mirror' to be able to jump in and out of groups. Annoyingly, every time I leave the mirror repeat layer, the background is automatically unlocked without me interacting with it at all. This causes me to accidentally drag the layer and require it to be locked again. 

Any ideas?