This is why it's advised to align the control sector of a sloped FOF with the in-map sector containing the FOF. The slope is pinned to the linedef defining it -- the platform technically is tilting properly, just relative to the control sector instead of the actual map space.
Make sure the slope-defining linedef is on the same line as the in-map 'pivot' linedef. An easy way to do so would be to use Draw Line mode to temporarily extend the pivot linedef to OoB space, then moving the control sector to align with that extension. (Be careful with it, however!)