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If you lock AF on a dark area and recompose, metering is also locked at the moment of focus and thus it renders the dark area it as a medium tone resulting in overexposure. Depending on your preference, you could do one of two things: 1. anticipate that the dark area will fool metering and dial in appropriate EC or 2., use the AE lock button (*) to lock metering from a medium toned area, recompose and focus as you please. I prefer to dial in EC as I can guess very accurately. In olden times I used to lock the spot meter on a medium tone in the same light as my subject to get my metering and recompose/focus. However, estimating EC is much faster for me.
And, finally, although we all wanna love autoISO, it pretty much sucks and often causes more harm than good. I'm amazed whenever I want fill flash in Av mode, whether in blazing sun or a candle lit room, autoISO only sets ISO400...