When the file saved is 450-501KB, it'll save as null 75% of the time.
Anything higher or lower saves correctly 100% of the time.
To avoid this problem, I have to use Hangouts. It seems immune from this issue. Every other third party app, Chomp SMS, Handcent, Go SMS, Textra and Hello SMS has the problem.
I know for sure this issue happens on SlimKat ROM and AICP ROM with stock kernels and ElementalX/Franco kernels.
EDIT: Issue present on CarbonROM now too. Gonna test these ROMs again but with no apps installed other than the thirdy party app.