This is exactly what you expect to happen. The open hot tap(s) run while the system depressurises then slow(s) to a stop at which point you follow the instructions and hold the TRV open to break the vacuum and let the air space fill as the water drains under gravity.I have a megaflo unvented cylinder on the first floor of my property and I am trying to recharge the air gap. I have turned off the incoming cold mains valve upstream (and right next to) the pressure regulating valve and then opened the hot water taps at the lowest point in the house. The problem I am getting is that I have had the water running out of these taps for at least 10 minutes with some really loud gurgling noises from the tank while the water is running. The hot water tap flow reduced significantly and did actually stop for around 10 seconds thinking it had worked (so I can then turn the TRV valve on the side of the tank to recharge the air gap), but it suddenly started to emit more water and again didn't stop.
Unless you have plumber's hands, wear a glove on the hand you are holding the TRV open with because it has a strong spring, sharp knurling and you'll need to be there for up to 20 minutes for it to complete draining.