I don't have experience with beta-flight, but falling down copter and disconnecting receiver sounds like fail-safe.
When there is a problem, fail-safe kicks in, which is usually set to no-pulse and copter falls down to avoid fly away. This is the right thing to do, other fail-safe options are usually dangerous.
It could be that you exceeded range of your transmitter (flying too far) or you have a bad antenna, or you have other strong transmitter that interferes with your receiver.
For example WiFi works on 2.4G and 5G, my camera with WiFi used to interrupt my 2.4G FrSky receiver, disabling WiFi from camera solved the problem.