Photo Library from Time Machine backup not recognised in new MacBook Pro
I've been using a MacBook 12" for a year or so and decided to upgrade to a MacBook Pro. So as I've done in the past with all my MacBooks, I've used Time Machine to migrate everything over. This involved backing everything from my MacBook 12" up on an external hard drive (the same one i've used for all my backups prior), and plugging it into my new MacBook Pro to migrate everything over.
Then I erased the disk on my MacBook 12".
Now moving onto my MacBook Pro (using MacOS Mojave), I realised that none of my photos are in the Photos app. I even tried to migrate everything again a second time using Time Machine.
However when I plug in the hard drive where all my backups are, and navigate to the date where I last backed up on my MacBook 12", I can see that my photos library is still there. When I double-click on it, it gives me the message "Cannot Import Items. These items cannot be added to your Photo library because the file format is not supported by Photos."
I'm not sure what is going on. I've checked on Disk Utility, the external hard drive I'm using does say "Mac OS Extended (Journaled)" whereas when I click on my internal Macintosh HD, it is listed as an "APFS Volume". Does this have anything to do with it?
MacBook, 10.14