This appears to be confirmed by JER not listing any sources for Frozen Cores (while other drops all have sources listed), and the fact that I creatively tested a stack or two of snow golem spawn eggs with a looting sword and only got snowballs.

I can't rule out that this isn't a cross-mod interaction of some description however. I'm not sure what method you're using to inject loot and it's possible that there's an issue with the null-pool-name addition that plagued grass injection for a long while -- potentially caused by another mod. I'll try to dig through the server logs to see if there's anything of note.

Thanks for reporting this. Apparently the loot table name has changed from snowman to snow_golem so I had to fix that. Now the frozen cores drop correctly. New version is on CF, should be approved shortly.

Awesome, thank you!