Terminating unused inputs and outputs
If you need an AC coupled 50 ohm termination to a 50 ohm microstrip line, then once you've passed through the 50 ohm termination resistor, you should be at ground.

Consider the resistor then capacitor case. After the resistor, you will have the physical length of the capacitor, its ESL, to go through before reaching ground. That will mean the termination will be higher impedance than 50 ohms.

Now consider the capacitor then resistor case. The capacitor can be placed in the line. Any excess inductance of the capacitor body can be matched out by increasing the size of its pads, to create a PI low pass filter in the line. With care, the line stays 50 ohm impedance all the way through the capacitor. Now put a 50 ohm resistor to ground, or better still, two 100 ohm resistors in parallel.