Why AEL does not work in ISO Auto ?

[didman69]

My favorite shooting mode would be to manually fix speed and aperture and let the camera automatically choose the correct ISO.

But of course, I want to be able to select an exposition measure and then recompose, which requires a lock of the exposure (AEL).

Unfortunately, it seems X-T4 (and probably all other Fujifilm cameras) is unable to do this. AEL is not available when speed and aperture are fixed.

Does someone know the reason of this limitation ? I found the same weirdness on other brands (Panasonic, Olympus, Nikon) : on manual mode, AEL is unavailable, even if ISO is in Auto mode (it's obvious why AEL isn't necessary in full manual mode, but not so clear as long as ISO is turned Auto).