Only Satechi bluetooth keyboard extension is recognised during log in (FileVault on)

Hi!

I have an iMac with original keyboard. I bought a numpad extension from Satechi that matches perfectly with Apple Keyboard. Both are connected via Bluetooth.

The problem is that, after connecting the Satechi keyboard extension, during the log in process, the original keyboard is not recognised anymore. I can't type my password. The only way is to connect it by cable. The strange thing is that the Satechi keyboard works perfectly fine on log in screen. I can not use it to enter my password because I have only numbers on this keyboard.

FileVault is on, I heard there are some problems with it preferring only the original keyboard, but in my case is the opposite.

I tried turning on the iMac with Satechi off and original keyboard on, tried with both on and both off, same result every time, no bluetooth connection from original keyboard, works only by cable.

The only way I can make it work as it should is to unpair Satechi or to forget and pair again the original keyboard after pairing the Satechi first. The second option works for 2-3 days and then, somehow the Satechi is recognised as primary keyboard during the log in process.

Is there a way to tell MacOS or FileVault that my original keyboard should always be my primary keyboard?