Exposure preview shift when loosing focus

[Pcoloma]

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Hello, I’m not sure this is a bug or something I have been doing wrong with my q3

i’m trying to make a close up (but not macro) of an object in subdued light (dark interior). 

I’ve set ISO to 100 not auto, AF to Single Afs, multi view metering, no exposure compensation. Aperture and speed auto. Exposure preview set to PSAM

When I press the shutter to get the focus, and the square brackets go green the exposure is spot on (no screen flash in the EVF)

On the other hand if the focus fails (red) I get an exposure flash of +2 or +3 exp. The scene flashes once bright, but if I refocus correctly then all goes back to normal.

have someone experienced something similar?

many thanks