I have a new iPhone 12 Pro Max and when I started deleting text messages off my previous phone the deletions also occur on my new iPhone 12 Pro Max. How can I delete the text messages from my previous phone and preserve them on my new phone?

It sounds like you have elected to house your messages in iCloud. Otherwise, deleting the messages from one device would not affect other devices.

Once you delete iCloud messages, they are gone forever.

How to delete messages in iCloud - Apple Support

You have setup iCloud for Messages on both iPhones. This is fine if you wanted to keep both iPhones in synch. You want your old iPhone to be independent from Messages in your new iPhone. To do this, get your old iPhone and open Settings ➔ Apple ID (Your Name at Top) ➔ iCloud ➔ Messages : Ensure that this is set to OFF.

Once you do the above then your old iPhone will be out of synch with your new iPhone in relation to the Messages app.

Axel F.

If your plan is to give away the phone, there is no need to go thru and manually delete stuff.

You should sign out of your Apple ID and erase the phone

What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support

go to Settings > [your name] > iCloud and check the switch position for Messages

with the behavior you are seeing, this switch must be turned on.. you cannot recover your deleted messages.

you could recover messages if you have a backup that contains them.. but because that switch is turned on, Messages are not contained in an iPhone backup. And I doubt you turned off the switch, performed a backup and then turned it back on.

So probably not the answer you want, but your Messages are gone.

Now.. on to the old phone. What do you want to do with it? Are you going to keep it or sell it/give it away?

Click on this link -> What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support and follow the instructions prior to giving the phone to your sister.