I had a '94 4.0l with similar problem, disappearing coolant with no pressure loss on test.
It was the overflow hose, had a small crack where it ran over the top of the rad, coolant only leaked out when engine was hot, hose was being used.
And location caused it to evaporate pretty quickly.
Also if wondering about a possible head gasket leak or just as a test next time you do a pressure test.
Install pressure tester and pump it up to 18psi
Disconnect coil or all spark plug wires
Turn engine over, if there is a head gasket issue then gauge will go up and down on compression and intake strokes of the leaking cylinder.
To find the leaking cylinder, remove 1 spark plug at a time and crank engine, when gauge stops jumping then last plug removed was in the leaking cylinder, reinstall plug to confirm.
You can do this same test without a pressure tester, it is called the Glove test.
Put a latex glove(with rubber band) or balloon over rad cap opening.
Block off overflow hose.
Cooling System is now sealed.
Disconnect coil and crank engine.
If glove bounces then there is a head gasket leak(or cracked head)
Pull 1 spark plug at a time to ID cylinder that is leaking