Keith David Bershatsky wrote:
> Perhaps there is something that may stand out (to a trained programmer) in the 01/31/2019 commit ....

It did change the buffer struct layout, so it's a good candidate for a culprit.

For what it's worth, I cannot reproduce the problem in a 32-bit build under 
Fedora 29 x86-64 (GCC 8.3.1). I configured this way:

./configure CC=gcc -m32 -march=native --enable-gcc-warnings 
--without-imagemagick --without-gif --with-modules --enable-checking=yes,glyphs 
--enable-check-lisp-object-type --with-gnutls=no

and built Emacs master with the attached patch x.diff.

My guess is that the problem has something to do with the unportable assumptions 
we've long made about struct buffer layout. I am attaching ptype.txt, the output 
of the GDB command "ptype /o current_buffer" that Eli suggested; please compare 
it to your ptype output.