Yesterday I deleted many messages and large attachments from my iPad. I thought the same messages would also be deleted on my iPhone ? They are still there !!!
How to delete messages on iPad and iPhone at the same time.
iPad Air, iOS 11
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Ensure the Messages in iCloud is enabled for both your iPad and iPhone:
Settings > [Your Name / AppleID] > iCloud > Messages - set to ON
This setting will keep your devices synchronised; deletion of a message from one device will delete from both.