100% sure - I can see both on the master and the satellite node they are connected (and all zone checks are passing).
I ended up stopping the master, renaming the icinga2.state file and restarting and everything has come good.
I realised I encountered this same issue in 2020, so I ran the same troubleshooting steps and deleting the state file resolved it.
Is this something I should raise as an issue in GitHub? I’ve had a look through the source code and it appears that the master should be handling checks that have a future timestamp, but in my case I was only seeing that the “log position” was three months into the future.