@aglet: I'm positive that it's not Lightroom because I've noticed 'jump' already on the preview on the camera.
@aquadome "The other possibility is that the light actually changed over time, even if you didn't notice it with your eyes." It was getting darker. I started soon after sunset. That explains why it was getting gradually darker. However it doesn't explain the sudden jump. It could be some lamp turning on that I dodn't notice at the time but I am pretty sure that it wasn't the case because I was taking pictures on a beach where there was only one lamp nearby - it was alway turned on.
Originally posted by KjellAV You have to set this in the menu. Change the WB Adjustable Range to Fixed. If you dont do this, the WB will do some fine adjustment even if you have manually set the WB.
KjellAV
Ha! Indeed theres such an option! Looks like this must be the correct answer. I need to check if it works on the nearest occasion.
However I'm still kindof worried because on another occasion when I was doing timelapse (it was jpgs then) at some point an obvious jump in exposure happened. Pictures got noticably brighter even though I was shooting in Manual. Still might have been the "WB Adjustable Range" feature.
Thanks for all your answers. I'll keep you posted guys.