The difference in the offset is based on differences in the total size of the head and body in the two builds.

Yep - thanks ikegami and dave_the_m.
In the course of pondering the reason that #18363 was afflicting only x64 -Duselongdouble builds, I was idly doing some Devel::Peek::Dump() operations when I noticed the puzzling behaviour.
I couldn't think of a reason that the SvIV should be exhibiting the differing behaviour between the two architectures ... but, as you've pointed out, ptrsize will change. (Another one I can think of is sizesize.)

Cheers,
Rob