Hey there, bevchris.
Welcome to the Apple Support Communities! Thanks for posting about iMessage conversations not syncing across your devices when items are deleted. We'd be glad to help.
Are you just using iMessage on your devices or did you enable the optional Messages in iCloud feature? This is important as that feature is what syncs the removal of the posts. If both of your devices are signed in to the same iCloud account, the messages would come in to each device, but to sync removal, you'll want to make sure that Messages in iCloud is on, as that will sync them with all devices. Check out Keep all your messages in iCloud to make sure that feature has been enabled, so that you get the behavior you're wanting.
If that feature is already enabled on both devices but isn't syncing, you might want to Restart your iPhone and Restart your iPad to see if anything changes. If not, verify they both have plenty of available storage. More info on that in How to check the storage on your iPhone, iPad, and iPod touch.
Let us know what you find out.
Be well