Hey B.I. Bailey: my 1999 Mazda/Ford B2500 manual single cab is doing the same thing but only in "A/C Max" position AND...when I accelerate hard. When on just the "A/C "switch/dial position, cooling is nominal and no bypassing out defrost vent (even with hard acceleration.) The issue you are describing happened to me X3 times and a broken or cracked vacuum line was always the cause, but like you said the lines were good on yours. The issue I'm having now is a real problem since I have not located a broken or cracked vacuum line (like your situation.) I still have A/C though but only in A/C dial position. Does it bypass on the A/C Max dial position AND the A/C dial position, or just the A/C Max position? The vacuum actuator is up behind the glove compartment (at least on my 1999 Mazda/Ford.) I think I'm going to replace the actuator pump which is the vacuum operated cyclinder piston thing that moved the vent/door. I don't think I have a vacuum reservoir (the round ball/bottle,) at least I cannot find it unless it's under the shroud under the passanger side fender. I would just replace it all: reservoir, and actuator pump.
I'll re-post once I replace the actuator piston. I attached a few pics of vacuum actuator pump (which opens/closes the vent door up in the dash.)
Update Oct. 6, 2019: I just ordered the vacuum actuator cylinder pump (connected to the mixer door) I will get to this soon enough, but the question is still: does the 1999 Ranger or Mazda b2500 have the round vacuum reservoir tank (or is that on the earlier model trucks?)
Update Nov. 24, 2019: I removed the radiator & wiper fluid reservoir to see things better. Also, removed the inner plastic fender liner and found the vacuum reservoir (see attached pic.) It was in between the inner fender and the plastic liner...sheesh. Must be a Mazda distinction. Also, sure enough, found a broken grey vacuum line which may be the issue with bypassing to defrost on acceleration (in A/C MAX only.) I will hold off on replacing the vacuum door actuator until vacuum like is fixed to see if it fixes it.