It works like this:
If the number of mandatory specials is at or above maximum, only mandatory specials will be generated. If they can’t be generated on current overmap, they will be generated in nearby randomly selected ones.
If the number of mandatory specials is well below maximum allowed, non-mandatory specials will be generated in free spots.
The mandatory slots are wasted on stupid shit like motels (2 minimum occurrences - to what end), like 5 campsites, cemeteries, road stops and so on. Once the maximum is reached, ALL non-mandatory specials are always dropped and never considered, then current mandatory specials stop being 100% mandatory and turn into “kinda important”.
You could PR a drop in mandatory specials for things that shouldn’t be mandatory. Or you can keep working against the system by bumping mandatory specials above maximum, but then expect it to behave badly at times.