No worries, Rob! 
And Dorico is correctly interpreting them that way too. If you set the padding and minimum width to 0, the C and its boundary box is clearly more narrow than A. There doesn’t seem to be an obvious answer as to why Dorico thinks there is room for the A but not the C in leo’s example. There must be another factor in play here too, unless there is some sort of bug in the spacing algorithm.
Just guessing, I wonder if Dorico is including any side bearing or kerning that would be present in strings with a space character either side of the rehearsal mark text.
That might be a good typographical idea for text enclosed in a box, even if you change the amount of padding from the default size of a space.