Have had this happen often using the PMDG. Will set a frequency and then it will switch. I think this has to do with my key bindings for switching views in the cockpit which is set with shift+# and if I hit the number key wrong it will “switch” to the frequency that is representee of what 1 would be in the MSFS ATC window. When flying online with ATC I have to make sure I am extra careful with changing views. It’s annoying but I just need to go and review my radio buttons or change my view bindings.