Flat signal when rescaling to Volts

My signal is in uV(microvolts) and when i use raw.apply_function(lambda x: x * 1e-6) and plot i get a flat signal, But when i use the raw.plot(start=100, duration=5, scalings = 'auto') i get a normal plot. I saw other discussions and i’m confused, does MNE automatically rescale the units to Volts?

what is the input file format?

if your data are in uV eg in .edf then mne will convert it to V on read.