SL2S Exposure lock not working on Continuous Shooting

[Ken Abrahams]

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It’s taken a few months and every now and again I have been getting over exposed images on the SL2S with my SL 35 APO lens. At first I thought the toggle focus point was the error. A human error. But recently shooting in continuous mode I have seen the exposure change in viewfinder.
It changes one stop. Going back over the shots in Lightroom I can see it too. The camera hasn’t moved and remains on subject and there is a huge change of shutter speed in the middle of my burst.

I checked if the same error is on my SL2 and with SL 50 Summicron and APO 35 Summicron and there is no error, no change of exposure while on medium and or fast continuous mode. So after six months of using the SL2S it seems it has an issue, unless I am not doing something right? 
 

# changed AEL and AF lock to either toggle and shutter button and it doesn’t seem to hold it’s position (shutter speed) after the first exposure in the burst. 
 

Any clues to this errant behaviour would be appreciated, Merry Christmas all,

Ken