Had a similar issue on my zombie map. Make sure that you have the “Violence” questions answered appropriately. If there are fiend/zombie antagonists, then you’re going to have “Violence against humans” (them attacking you) and “Violence against non-humans” (you attacking them). Most likely “frequent” and “close up”. The violence category also applies to environmental damage according to IARC, so like death runs and survival maps.
My speculation: if you have any implied violence (in name, thumbnail, or description) and wind up with a rating less than “Teen” they’re going to reject it.