All autocorrects turned off, wasn't happening few months before. Sample - write sentence "That would be". Put cursor after word "would", start deleting letter by letter. When there's only "That w be" remaining, text suddenly changes to "Thatul be". If I try to correct such cases (especially in longer sentences), usually I'm getting some other meaningless replacement, to the point where I'm unable to correct at all without rewriting everything from the end. That's insane.
Further in the sample, autocomplete options still show choices "wouldn't would would be". If I select "would", final sencence becomes "Thatuld be". Which looks like replacement indexes are getting corrupted, in addition to the autocorrect which had to not happen at all.