Compiling the same source without proposed enabled:
ubuntu@Mcdivitt-B0-Cartridge37:~$ ./int64test nan
result 0 status 1
FWIW, ./int64test NaN behaves like disco-proposed always (on arm64...), and symfony has special code handling "NaN", so a reasonable expedient fix is probably to patch symfony to not be case sensitive there.
Given that this only seems to happen on arm64 and not, say, amd64, my guess is that it's also an ICU bug but ICU is pretty confusing and documentation on intended behaviour of how NaN should be handled by number parsing seems to be non-existent.