keyboard needs pairing after shutdown

New to Mac. So far so good. I just have a question about bluetooth keyboard. Magic keyboard (batter type) . New batteries. Battery level showing %100. Keyboard stays paired during daytime use. Even in long sleep mode. But if I shutdown for overnight I will need to pair the keyboard again. I bought a used iMac ( mid 2011) and a used keyboard /mouse. Mint shape. Looks like last release of keyboard/mouse using AA batteries.

I can remove keyboard from bluetooth settings. Shutdown computer. Start computer and it will go through pairing to bluetooth with verifying keys.. Hoping that would correct issue. But If I just shutdown overnight when I start up computer. I do not see bluetooth wanting to pair to keyboard. Keyboard just does not function. Tried just hitting enter key hoping it would work. But I end up power failing keyboard off/on. Bang. Screen shows image of keyboard stating keyboard connected. I DO NOT have to key any verification keys as would if being paired from scratch.

Thanks

Lee