Hmm mine acts a bit differently than that.
Like I said, sometimes it safe homes, other times it just homes in the corner.
If it homes just in the corner, the 9 location grip that it tests for leveling will start more toward that back corner.
If it safe homes in the middle of the bed as it should, then the 9 location pattern is centered along the bed.
I have never tried running 2 G28 commands before the G29 command though.