It’s not clear from your post as to what is happening. I’ve just run a test with CSV in FFT and screen output modes and everything look fine to me. When you view a saved CSV file, it should contain the same min and max frequency (in steps dependent on the selected CSV mode) calculated when the centre frequency and span are set.
A screen shot and a copy of the CSV file in screen mode would be most useful. The screen shot would particularly useful, as I can then see all settings at once.