[Q]

It seems to be a problem connected to the reload time of the main weapon:
I repeated the test with an attacking ship that has wave motion gun and PDC. The ship fired its WMG at range 8, but during reload of the WMG it attacked the fighters with the PDC at range 5. When the WMG was reloaded it moved again to range 8 and fired! Not very clever, but it obeyed the order of "maximum range" if you consider that during the WMG reload the PDC were its only weapons available.
But if you use an attacking ship with WMG, anti proton beam and PDC? The ship will keep a distance of 8 fire the WMG and anti proton beam but not use the PDC.

Therefore you have two options:
1.) As already suggested don't include PDC's in the design of the ships with high range weapons and maximum range strategy.
2) Include at least one weapon in the design with a reload time of 1 that has a high range.