There will always be lumen loss the larger your grow room is, whether it's up, down or at the sides.
Most need that room above the lights to eventually move the light/reflector up, if you don't plan on moving your light up during the entire grow then one needs to build the proper sized grow room.
I have a 4x4x6 1/2 high veg room and a 7x5x7 1/2 flower room and my lights come WAY DOWN in each room because you need that flexibility in up and down distance to adjust the height the lights are form the canopy.
So in theory yes you lose lumens but it's minimal loss if you have flat white painted walls and ceiling, panda film, mylar or some other reflective surface which you should have. Plus, it's necessary to have that space.
In the final weeks in my flower room my reflectors are all the way up to 7 feet, usually with only maybe 4" of space to the ceiling.
It's normal. A higher ceiling can also help with heat.