In page-lay.mkvi I see, at lines 1591–1594:

\definepapersize
  [doubleoversized]
  [ \c!width=\dimexpr \paperheight+\layouttargetparameter\c!distance\relax,
   \c!height=\dimexpr2\paperwidth +\layouttargetparameter\c!distance\relax]


I would expect to see:

\definepapersize
  [doubleoversized]
  [ \c!width=\dimexpr \paperwidth +\layouttargetparameter\c!distance\relax,
   \c!height=\dimexpr2\paperheight+\layouttargetparameter\c!distance\relax]


--
Rik