Time Machine won't stop asking to inherit a backup

There was a similar thread based on this (almost same subject line) but it was in 2015 and so I think things have changed - and now the solution there doesn't apply. I have two computers, a mini and a MacBook Pro. The mini is connected directly to my external drive and the MBP backs up via the network to it.

My mini asks every single time it does a backup if I want to inherit the MBP's backup or create a new one. I always have to say create a new one, but it doesn't actually create a new one it just finds its old backup in the backups.backupdb folder. I know this is still keeping the two separate because when entering Time Machine on either computer they still have unique differences. The MBP is my first computer and when I got the mini I used the migration assistant to copy over all my files. I'm wondering if this has something to do with it.

I noticed that the MBP has it's own .sparsebundle but the mini doesn't - not sure if maybe that is causing the problem or how that happened.

I JUST started using Time Machine again and had wiped the external drive of all backups previously (I kept on some wedding videos etc). Any help would be greatly appreciated.