I suspect it's a Math::BigInt/BigFloat bug
My suspicion was correct.
I dug into it a bit, and came up with
this Math::BigInt PR, though I don't know if that's the optimal fix.
As mentioned in that PR, I subsequently noticed that there did already exist other builds of perl where ivsize=4 and nvtype was __float128, and that they were also striking the same Math::BigInt test failures as I was seeing.
(For example,
https://www.cpantesters.org/cpan/report/2737c762-6c8c-1014-a5db-5bbf06c191b3 )
There has been no response to that PR yet. I'll file an Issue against perl5 itself if there has been no response to it by the end of next week.
Cheers,
Rob