You are right, there is something wrong with htmlize. My code can pass all the random test. If I use htmlize, however, it will crashes.
I stuck in the 11th should_work_for_a_single_glider_over_all_4_repeating_generations as well.

funny cheat :-)

This is some cheat to pull out all of the data to guess the formula

Cheating!

This solution won't work if field is filled with "5" (for example).

This solution won't work if entire field is filled with "5" (for instance).

There is an issue: initial template cannot be compiled as size_t needs to be defined (so at least <stdio.h> needs to be included)

the crash in your original message is normal, as per the comments in the initial code:

NOTE: Please make a deep copy of the GoL universe and modify that instead
of the original universe cells passed in because the test cases will
free the memory allocated by your solution AND to the original grid

so, returning cells triggered an error known as a double free() where the same pointer is freed twice

On the other hand you are not the first one reporting issues in C for this kata; I will try to investigate a bit and add better error messages if I can

Use a spoiler flag please. I see no crash in this, just an answer not accepted.