why did my messages stop syncing with my iphone?

Are you signed-in to iCloud with the same AppleID on both your iPad and iPhone? You would be well advised to verify that Text Message Forwarding is explicitly enabled for your iPad on your iPhone.

On your iPhone:

Settings > Messages > Text Message Forwarding - set to ON - and explicitly enable your iPad from the list of devices.

Also ensure that Messages in iCloud is enabled on both your iPhone and iPad:

Settings > [Your Name / AppleID] > Messages - set to ON

More information:

How to forward SMS/MMS text messages from your iPhone to your iPad, iPod touch or Mac - Apple Support

Fault finding if needed:

If you can't send or receive messages on your iPhone, iPad, or iPod touch - Apple Support

Hi there altoal,

Welcome to Apple Support Communities. We're happy to help with the messages not syncing between your iPhone 11 Pro and your iPad Pro. First, we'll need to confirm that Text Message Forwarding has set up properly. Follow the steps here to check:

How to forward SMS/MMS text messages from your iPhone to your iPad, iPod touch or Mac

Let us know the outcome, and we can go from there.

Have a great day!