fixed layout epub exported from indesign with image over two pages (spread) appears with scroll bar

Bob's dealt with the technical issue, but let me throw in the two basic caveats: fixed-page EPUB (FXL) is a specialized and largely deprecated format, suitable ONLY for "picture as a page" books. Which, to be fair, it sounds as if yours is. And to further clarify, FXL is a very poor choice for "text" books although many novices choose it as what seems to be the simple and obvious option, like PDF. (It's not. Text books should be reflowable EPUB.)

The other point is that EPUB is highly dependent on the reader. While there is an option/fix for your specific problem, you are at the mercy of whatever reader every user chooses to use, and hardly any two of them display pages in the same way. You may well discover or get complaints that your book displays poorly or awkwardly for another user, and there is little you can do except suggest they use the same reader you have found to work correctly. (If that's iBooks, fine, but Apple's reader is not quite standard and books that appear properly in it may not appear correctly in a more open-market reader like Calibre or Thorium. You are thus limiting sales and a satisfied user base to the Apple ecosystem.)

But good and simple solution to a problem here; that's just not common with EPUB export!

ETA: one of the faults of fixed page, especially with converte spreads, is that screen sizes are only so large. I'm not sure zoom and pan is an asset when trying to view art or "arty" pages. The primary rule for book design and publishing, to me, is "respect the medium" — and it's a fault, I think, compounded/enabled by most FXL layouts, to try to make digital pages replicas of print ones. What works in a coffee-table format doesn't work, really, on even the largest common tablet screen.

What reader are you looking at this with? Is that a two-page spread converted to a single page on export?