I don't use any of this type of equpiment... So my comments will be for generic information I've read and other common causes I've seen or heard of only. That is in part of why I mentioned the protocol requirements earlier since you gave no additional information on what other equipment you were using so had no other direction to suggest first. In short, it was and still is the first place to look. Again, my suggestions will be for common areas that mistakes are often made in or things often overlooked if I know of them.
If you were able to bind the NX8 to the 4650 and then only were having problems with using betaflight... that indicates a problem in your betaflight setup itself, either wiring or settings in the software. I've seen a lot of users have problems that were simply from using an incorrect USB cable that was power only and didn't support data, so check that first for that configuration. Another software mistake is in the selection of the COM port being used must be the correct one in betaflight. This radio/receiver may be the best direction to get help troubleshooting since you are able to bind using that combination so have one less thing to diagnose.
Triple check your Radiomaster TX12 setup is actually correct. If so, check to see if they have released a firmware update and if so, update it ( I'd do this if available anytime on any radio ). If you do update the firmware, this will usually reset the settings and they will need to be reset once again. After that, try removing the connections to the flight controller and just give the receiver power and try binding again. This helps to eliminate possible wiring issues between the receiver and FC as a potential cause. If you still have problems binding, it's probably time to actually contact support for the receiver directly to see if they have any additional suggestions. When contacting support, tell them what your exact equipment is that you are working with, if you updated the firmware etc... and any things you've already done or tried to help speed the support process along so you don't get recommendstions that you may have already tried or done. A lot of support people are entry level employee's that will often respond with a default set of suggestions, even ones you have told them you've done... just be patient with them and reply accordingly stating the steps were performed ( even if you had told them already ). Of course, you could get lucky and get someone that does know what they are talking about the first time through, just don't assume it. Once, and IF it does start working and you get the binding to work... you still have to work through the Betaflight issues.
When setting up Betaflight connected to a PC running Windows...
Make sure your audio/speakers are on and turned up where you can hear them. Then, on Windows, when connecting a USB device with a proper USB data cable for setting up using Betaflight, Windows will make the USB connection sound ( by default, it's a 3 tone sound that is played ). This tells you that Windows has seen something connect but it doesn't tell you if it ( Windows ) was able to install the correct driver or not. You can check the Windows Device Manager and see if any of the devices show a ! in a yellow triangle or not. If you don't see that, the driver is likely correctly installed ( this isn't a guarantee, just a good indication ). I believe Betaflight will list a COM port connection for one of eight ports, COM1-COM8... unless this has changed and I didn't see it mentioned before but if it lists more, then you have more combinations to test. I'm uncertain if you have to close and reopen Betaflight after changing the COM port setting or not... assume you do even if it takes a little longer to test each port available.
At this point... I think I've mentioned everything I can think of but there are a lot of others here with more info concerning Betaflight etc.