It's very simple and not a mystery at all. Rosemary prefers a sunny location. Purchase a healthy plant from a reputable store and plant it immediately. Do not forget about it.

It is difficult for a plant to recover after that kind of abuse. Buy healthy, plant it right, and give it proper care.

Not a mystery really, it kind of goes into hibernation if conditions aren't optimal, but not bad enough to kill it. My rosemary that lives inside my patio door with only a couple hours of direct sun in the winter does the same thing--doesn't die but doesn't thrive either. And that works for me, since it will go out again once the temps warm up. The two that I have under lights are much happier, and do grow a bit in the winter.

I think the problem with your first plant is that it was used to shade, then sticking it in direct sun (in June) shocked it. It needed to be hardened off to that much sun, similar to new seedlings raised under lights, after the three months of shade conditions. I would plant your new bush in a sunny location.