Caused by 25237d2625, when the new "template" mesh created
in the modifier started to use the proper default value for CD_ORIGINDEX
layers rather than just zero. Zero isn't correct because it refers to
the first element, not "no element". For that we need to remove the
original index mapping arrays completely. There is some gray area
about whether that is allowed too, but it's best here to just keep the
old behavior working for now.
d9398bb53f