Thank you Chariri and Alexei for helping out, especially for introducing me to ftinspect. The demos were extremely helpful and I certainly agree that FreeType is much better than what Windows can provide.
What I was curious about was why Windows selectively hinted SimSun 3.03 at 24px, but didn't do so for SimSun 5.18. (3.03 is the version supplied on Windows XP, and versions later than 5.00 come with recent Windows starting from Vista.) Both versions have identical "Fixed Size" tables when inspected with ftinspect.
The issue wasn't related to any Qt libraries, as they are depended on the OS to render font (as expected). (It might be helpful to note here that I used the QFont::PreferFullHinting property, which is not turned on by default when using QFont.) I probably should also say that it is also not heavily related to FreeType, so only continue if you are interested in some weird Windows ClearType and font metadata goofiness.
The reason for what I am experiencing is related to an interesting metadata section contained in font files. I discovered this when opening this font in external font editing software, and it is simply called "Hinting" in Hi-Logic FontCreator and "Grid Fitting" in FontForge. Basically, Windows adhere to the sets of rules in this property in deciding whether or not to apply hinting.
Grid Fitting Properties for SimSun 3.03
Grid Fitting Properties for SimSun 5.18
The limit of size 24 included in SimSun 3.03 caused ClearType to somehow be activated when I render glyphs onto an image with preferred hinting turned on. The limit is apparently adjusted for later versions, resulting in different behaviors. The numbers of embedded bitmaps don't differ. The images I included in my previous email were straight-up FreeType vs. ClearType.
As I previously mentioned, I don't have any experience with the internal structure of font files, so it took me time to even find this table. I am not sure if this information is accessible by any FreeType libraries (it's also probably not that important to most users), but please let me know if I am wrong again!
A weird but fruitful investigation in the end. Thank you all again for your help! Happy to be here!