Just thought of something to try...
You could try treating Myth with
patchelf (which replaces the older
chrpath), and set the rpath to $ORIGIN/lib32 or something, and put the lib32 folder from my builds next to it, so it will load these, and not the system ones. You will possibly have to
escape $ORIGIN, so that the shell doesn't mangle it.
Then use
ldd and
objdump -x Myth2 | grep RPATH or
readelf -d Myth2 | grep RPATH to verify it loads the new libs. If objdump literally prints $ORIGIN/lib32, you've been successful. You can also test objdump on aquaria32.
(Fyi, when compiling, I used -Wl,-rpath,\$ORIGIN/lib32 from ccmake, as CXX flag.)
Well... this is more random experimenting than anything else.

But one thing Myth2 and aquria32 do NOT have in common is that Myth2 uses system libs only.
Could be the problem, but i doubt it - it would mean the ubuntu packagers have messed up.
The HiB release ships its own libraries too, and nevertheless it
had the rectangles.