Amazon seems to have a policy of ignoring prefaces, forewords, acknowledgements, indeed anything that in a traditionally designed book would have "front matter" paginated in lower case roman numerals. I think you would do best to call it something other than "preface" -- say "Marker" or "A Thought" -- in the H heading, followed by your epigram in a conventional paragraph.
I have had pretty good luck uploading epubs to the KDP that have no "text" semantic (SRL) at all. Most such books open at the cover rather than at page one, chapter one.
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