SendKeys using app, only numbers received in phone

Hi, i'm trying to send string to the android window using application (simple c++);

The strangest thing is... when I send "123abc", in Android text box only received "123"
At first I think this is the problem with the keyboard (I used null keyboard), but after switching to several keyboards.. I believe scrcpy doesn't *fully* process the input generated by app?

When I typed normally.. no problem,,,, but when the typing is generated by app, only numbers received by phone omitting (a-z). Is this known issue? thanks.