Bacteria causes infections, not just high humidity. I can have snakes sitting in 100% humidity and they're perfectly fine if they just dumped a waterbowl.
If a snake has a respiratory infection, regardless of how minor, you can't just up the temps and humidity and think it's gone. It's an infection and will persist regardless of how much you try to cover it up.
If the snake is whistling, it may have shed in the nasal passages, the humidity may be too low, etc. Try upping the temperatures to a good 93-94 warm side and 82-84 warm side. These temps are good for year round. They shouldn't dip below 90 on warm side, 80 on cool side. Humidity 55-65, not under 50%.