Hi there AlexAshley09,
We can provide some guidance. If you have Messages in iCloud turned on in Settings > [your name] > iCloud > Messages, then all your messages are already syncing with iCloud. When you use Messages in iCloud, your messages aren't part of your backup since they're already stored in iCloud. Therefore, when you restore from a backup, it won't include all your messages when restoring. To access your messages again, you'll need to sign into iCloud with your Apple ID and ensure Messages in iCloud are turned on.
If you turn off Messages in iCloud on an iPhone, iPad, or iPod touch, your message history will be included in iCloud Backup.
Have a look at this link to learn more: Use Messages in iCloud
Keeping that information in mind, if you are signed in with your Apple ID and turned on Messages in iCloud after restoring your iPhone, it's possible they may still be downloading from iCloud.
We hope this provides some clarity.
Take care!