Yeah, it's not so clear-cut it turns out. My other iPhone which was OTA updated to each beta since iOS 18.0 beta 1, where it functions perfectly. Yet on a clean install of iOS 18.0 beta 5, it stops working after a reboot. A clean install of beta 4 does not reproduce the issue. So it's something in 18.0 beta 5. My advice is to those on clean install, do not reboot. I couldn't reliably fix it even by turning off and removing Find my from the phone and then reenabling the functionality. Sign out and in of iCloud fixed it temporarily, until reboot, but that's cumbersome.