Transfer between Android and iPhone

I just received an iPhone 12Max Pro. I've been using Android phones for years - most recently the Samsung Galaxy S9. I backed up my S9 to the AT&T Cloud, including text messages. After the transfer, text messages other people sent to me appear in messaging, but about 98% of my texts to them do not appear. It isn't a matter of me having had an Android and no iMessage account, because this is consistently an inconsistent issue with people who have iPhones and Androids. In other words, it doesn't consistently happen for all of my friends who have iPhones or Android phones. Some of my messages appear within text exchanges with a person (Android and iPhone users) and some don't. It's completely random. I did a massive clean-up of messages before backing them up so that only conversations important to me would be transferred. Now only half the conversation is there. :\