MacBook Lock Screen Message Not Updating

Hi everyone. I always set a lock screen message on my MacBooks (System Preferences > Security & Privacy > General > Set Lock Message) with a message saying something along the lines of: 'If this MacBook is found, please call [phone number]. Thank you.'

I recently updated my lock screen message because I had to change the phone number shown, and it's displaying on the log in screen below the password entry box when my MacBook is locked as expected, apart from when the Mac is first booted up (turned on or restarted), when it instead displays the *old* lock screen message. I thought perhaps this issue would resolve itself over time, but alas it has been a fair few days and the problem persists. Is there a way of forcing the message to update?

I have FileVault enabled for my MacBook (so my user data doesn't get decrypted until I enter my password for the first time post booting up) which leads me to wonder whether the new lock screen message is being stored in an area of my Mac that isn't accessed prior to this decryption, and the old message is still saved in an unencrypted area that is accessed upon booting up. As an aside, interestingly when my Mac boots up the wallpaper on the initial log in screen is the default Catalina picture, and not my custom desktop wallpaper (like it is for any subsequent log ins) - is this also a consequence of FireVault?

Anyway, this is something I really want to get sorted because if a good samaritan finds my lost MacBook and it isn't turned on, I still want them to be able to see my up-to-date contact info when they power it on. Any ideas?

By the way, I'm running macOS Catalina 10.15.7 on an early 2015 MacBook Air with a 128gb SSD, in case it makes a difference. I also have a firmware password enabled, although I don't see what effect that would have on this.

Thanks :)