but it's irrelevent to my issue

Why that?
I suppose it is the test with find_nb(24723578342962) (you should have given the input:-)
I don't have enough time to see your code, sorry!
For now I don't consider it is an issue: as you say lots of people passed the C kata... and lots and lots more the kata in all languages and they all give the same answer for this input value. If there were errors one can expect that someone would have seen them (24560 passed solutions x 104 tests or more for each).

Whe I run my code through the sample tests it failes me and the log I get says: "Expected -1 but got 4294967295". However, I checked the path my code takes in that specific test (by printing a different statement in every optional path) and my code returns -1, yet somehow the grader says otherwise.

and yes, I know 696 other people passed the kata in C, but it's irrelevent to my issue.

please consider viewing my code (I will post it as a reply to this comment if it's needed).

Thanks

It still happens, happened to me, too, exactly as you described.
I used malloc().
don't know if it helps you, but maybe another comment would help.