Bizarre copy of unknown

[Thomas Pfau]

I am having issues with my 5.16.2 build.  Modules that worked fine on 5.8
and 5.10 are now aborting with this error when returning an undefined value
by referencing &PL_sv_undef. I have been "fixing" these by changing them to
sv_newmortal() but I question why this code is suddenly broken. Does anyone
know what changed?