If gamma override is on, when entering a level, it has no effect, until I disable and re-enable it.
Besides, changing the overriding gamma value has no effect, until I disable and re-enable gamma override.

This is simply how it's implemented at the moment. So it's not really a bug, but it might still be worth it to change how things work to solve these small issues.

However the the first part doesn't sound normal. If there are no other mods or outside things changing the brightness value, it should remain enabled/bright through game restarts. Do you have Optifine or some other rendering related mod that might be changing the brightness value?