Not sure if intended, but it's bugging me to no end. It didn't use to be like this. 1. Spawn as engineer 2. Plop down a turret 3. Walk away from turret 4. Approach turret slowly while crouched while holding LMB (with the repair tool out) 5. Stop right when the turret's health bar shows up. 6. You aren't building it, it's too far away. Move a few units closer and it starts working again.
I distinctly remember it being the other way round - you would be able to repair/heal without seeing the health bar.