Annoying firmware "feature"

[khaksnes]

I have discovered an annoying bug in firmware 1.2.3 for 5D Mk iii.
I am trying to set up a custom mode to allow me good control and at the same time reasonably fast and safe operation. My custom mode is based on Av, Spot metering and focus only with AF-ON button. * button is set to exposure lock M-Fn to electronic level.
My intention is to focus, set aperture, measure light, if intended correct exposure, finally compose, checking the leveling as part of composing if appropriate.
The problem: Pressing the M-Fn button cancel the exposure lock. 
To me this looks like a bug as it  makes the electronic level nearly useless in many situations. I can't see any advantage of the current behavior.

[RobertTheFat]

Is that a change from previous behavior, or did the M-Fn button always cancel the exposure lock?

[khaksnes]

I am not sure if the problem is limited to 1.2.3 and 5D MK iii. I haven't tested it using older firmware versions.

I do own a 7D too but haven't tested for a similar problem there, if it is, it isn't likely to be fixed.