Apple A1016 Keyboard on an iMac Late 2013

I have an old Apple A1016 keyboard. This is a lovely mechanical keyboard and pleasure to type on. I have managed to connect it to my late 2013 iMac running macOS Catalina 10.15.7. To do this I needed to go into the Bluetooth in System Preferences and then turn on the keyboard where it is detected as "appletestappletest's keyboard".

The one problem I experience with it though, is that it does not connect to the iMac on boot-up. I have to connect another keyboard to my iMac and then go into Bluetooth and select connect. Where upon it connects and works fine. Does anyone know why this is happening is there anything that can be done so the A1016 connects to the iMac at boot-up?