I get the same need to join message for the second image.
Remember that a flash exposure is really two exposures in one: the ambient light and the fill flash.
If you really want to control the fill flash, then you should be shooting in manual flash mode that way you have complete control of the output of the speedlight. The camera will meter the scene and determine how much fill flash is needed for every shot. It continues to meter the scene during the actual flash burst. Unfortunately there is no flash value lock unless you shoot manual flash.
What was the ambient light like. I'm thinking that the light levels were low enough for the camera to believe that it needed to provide more light and fired at full or near full flash.
The issue with fill flash is that YOU as the photographer do not know the level of compensation that the camera is providing. If you want repeatable flash results, then you need to shoot in Manual Flash mode, which means running tests to know/understand how the flash behaves under certain lighting conditions.
If I were shooting that first shot (and I have no access still to your second shot), I would shoot in Slow Rear Sync...slow to record more ambient light. Hopefully your model will not be moving and blur the image, but the rear sync should sharpen her up and tell her when the exposure is done.
Take a reading in Manual mode, set accordingly (for example if the meter for a correct exposure in dim conditions was 1 sec at f/4, then I might shoot at 1 sec @ f/5.6 to slightly underexpose (keep the ambient level low) then set Manual flash at -1.7 (a good starting point for fill flash) and bracket my shots from there.