Join the club. And it's not just Pages on iPad, it's every platform. I typically jot down ideas on my iPhone, then transfer them to my MacBook Pro 16" with M1 Max for writing. And it used to be I could trust Autocorrect (and it's cousin Spellcheck) - which was especially true when writing a science-fiction novel, and you have alien names and places. And the first issues I noticed with Spellcheck was about a year ago (or so), where I could run it through the document, tell it that a name was spelled correctly, and it would just cycle through continuously, always highlighting the very words I had just told it to learn (possessive forms, like names with an apostrophe, are especially difficult).
And then I noticed the quirk in the past few months where Autocorrect would change homophones (same sound - different spelling/meaning) on the fly while I was typing, and that resulted in some unprofessional looking emails, or I wouldn't find it until I did another read-through. But if you turn off Autocorrect, it just sits there dumb and won't suggest alternate spellings. So it's one of those danged if you do, danged if you don't situations.
And ironically I think I uncovered the reason why via the AI Mode in Google Search. It suggested that it was because many of these writing apps, like Pages, MS Word, etc. have been training their Autocorrect/Spellcheck functions using LLMs. So you can blame it on AI for why your Autocorrect and Spellcheck aren't behaving themselves. And I've tried sending feedback to Apple 4-5 times now about the issue, but it falls on deaf ears. Although I do make sure to include the following indictment: Apple has a penchant for "improving the functionality out of their products".
So I hope this at least offers you a possible explanation. I would suggest giving Apple your feedback (type "feedback" into the search field and choose product feedback, then scroll down to Pages), however, like I said, that seems to fall on deaf ears.
Sincerely, David A.