You didn't say how the page breaks were created, so I can't speak to why they mightn't work on the KDP process.
I will say this--unless the files get kicked upstairs, to the "real" tech support, you may as well not ask the question. The people that you'll get, to answer a question like that, know nearly less than you do about making and building MOBI files.
Did you create those page breaks using page-break functionality in Word? Using page-break always or before, or...?
Hitch
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