I have a Nikon Z7 and when I use the Manual Mode to control both my aperture and speed (with ISO constant, which means that I removed the automatism "A") let's say speed 1/50 and aperture f2.8 (and making sure that the exposure scale shown in the LCD Display is good), I got a correct picture. But after I took the picture, the speed change and is no longer 1/50: it becomes 1/400, or another time 1/250 and changes randomly. Strangely my aperture set does not change.
So why my speed I manually set at 1/50 does not stay at 1/50 after the picture is taken if I want to make a second and third picture?
What makes this happen ??