Yes an alternator can still be bad even if the alternator tests good.
A vehicles alternator can still have an output charge of 14 volts but still the alternator may be bad and have a short in it.
I've had an alternator that tested good and put out the proper charging voltage and it was charging the battery but it also had a short in the diode since the diode went bad.
The bad alternator diode was causing a short in the electrical system and causing my alternators gauge to drop down.
The engine would start fine, then start running rough especially at idle and I tested the alternator with a Voltmeter and was getting the 14.4 Volts in normally puts out.
But the alternator was not supplying enough electricity to keep the engine running properly.
I've also had a bad alternator cause the battery to go dead overnight.