So I'm seeing the full credit now, and it may have been there sooner, I believe I was looking for it incorrectly.
Earlier I was going
to this page and looking at the
"Software / Video Games" section, which is
still showing $5. But the correct place to look is elsewhere (it's actually on every paid app page), see
this page for details. When I look that way, I now have all $8 in credit.
I suspect what is going on here is that the $5 credit from buying the game can be used either for the AppStore, or Amazon's digital software store. While the $3 credit is solely for the AppStore.
Hopefully this helps others checking their balance.
