The reason that the Gap to left of leftmost accidental option works differently for the first note in a bar than it does elsewhere comes down to precisely how Dorico’s spacing works.
At the start of the bar, there is some inviolable space to the right of the barline, and you can imagine that there is an invisible barrier that nothing can cross. This barrier is “infinitely tall”, so it extends both above and below the staff. The leftmost accidental then pushes against this infinitely tall barrier, and that’s what causes it to move two spaces to the right.
However, when the note with the accidental is being spaced against the previous note, there is no infinitely tall barrier in play: instead, Dorico is using a bounding shape that reasonably precisely follows the contour of the previous notehead and its stem. In the area directly above the notehead, there is nothing for the flat to push against, so Dorico doesn’t need to distort the spacing. If the note were one staff position higher, then the flat and the notehead would interact, and the note and its accidental would be pushed to the right.
Dorico doesn’t “know” that the previous note will end up with a playing technique when it is spacing the music, and the playing technique is positioned after the music has been spaced. This means that the playing technique has no choice but to go above the accidental. (At this stage, Dorico similarly doesn’t “know” that it is positioning the playing technique outside an accidental belonging to the following note or chord: it is working with a “skyline” profile and positioning the playing technique against that.)
I would agree that in this case you would definitely want to prioritise the playing technique above the accidental belonging to the following note, but at the moment that isn’t something Dorico can do.