I have the study going on three different iPhones. I managed this by restoring my 6S+ backup onto my 7+ and 8+. Now, no matter what phone I use to pair my watches with, it continues the study on that phone. Since my backup restored everything identical to the 6S+ the Apple study app uses the same email address so the number of contributions stay the same on all three phones. I have 6 watches so there are times I have a watch paired to all three phones.
My backups are encrypted so maybe that is why I was able to restore the Apple study to all the phones. Screencap shows my 7+ and 8+ with the app showing the current stats and start date. The stats are just a bit different because I wore the watch with my 8+ today so they haven't quite synced up yet to show the same stats.
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