The D3200 User Manual says:
"Selecting live view in "Auto" or "Flash disabled" mode enables
automatic scene selection (“scene auto selector”) when autofocus is
used." (Page 20)
Can it happen that you are in either of these, and not in P, A, S, or M?
Depending on the scene, exposure may differ from that which would be
obtained when live view is not used. Metering in live view is adjusted
to suit the live view display, producing photographs with exposure
close to what is seen in the monitor. In P, S and A modes, exposure
can be adjusted by ±5 EV in increments of 1/3EV. Note that the effects
of values over +3 EV or under –3 EV can not be previewed in the
monitor. (Page 101)
If in P, S or A, are you using exposure compensation?
Also, regarding Auto ISO:
Note that if the ISO sensitivity selected by the user is higher than
the value selected for the
ISO sensitivity settings > Auto ISO sensitivity control > Maximum sensitivity option in the shooting
menu, the value selected for Maximum sensitivity will be used instead. (Page 68)
So you might have locked ISO previously but have Auto ISO active this time?
Also, ISO Sensitivity for not P, S, A, and M defaults to Auto. Maybe you are using this setting but was locked previously?
Also, page 145 may be a good read in the manual.