Sentences change after I press send.

Predictive text probably uses a machine learning model. I've noticed that it learns expressions I use even if it got them wrong at first.

So long as a post on GN, which is not a 'formal' sort of place (mostly!), is comprehensible I don't think a bit of nitwittery from so-called AI really matters.

Predictive text is not obligatory. If you know you can do better yourself, you can turn it off.

I did a search for "Switching off predictive text" and the first result on the list was from Duolingo. It had instructions for Android and iOS devices.
support.duolingo.com/hc/en-us/articles/4403698695309-How-do-I-disable-predictive-text

Predictive text has a life iof its own
I read back but even then I still find mistakes 🙄

It is always happening to me and I do usually preview! Grr!

I nearly always preview before posting - easier than being annoyed by a mistake which the technology might have introduced. Sometimes it's hard to correct as predictive text seems to think it knows best!