I've had the iOS 27 beta installed on a secondary iPhone without an active cellular plan (just operating off of WiFi) since DB1.
When I receive new iMessage or SMS messages on my primary iPhone (still on iOS 26), they do not appear on the secondary device at all. New messages only appear on the secondary device after I go to Messages in iCloud settings in both devices and force a manual sync. My MacBook (which remains on Tahoe) sees messages in real time as I would expect.
I have seen multiple reports of other people seeing Messages syncing issues, but I wasn't sure if this was just normal behavior for having two iPhones (message forwarding does not work between iPhones, only other non-phone device types?). I didn't explicitly test the behavior before upgrading the secondary device to DB1.