Realms deletes certain predicate files for "invalid json," despite being completely valid and working fine in vanilla. I am not 100% as to the exact cause, but I would guess it is not updated to accept the new 1.17 predicate formatting.
Attached is the main datapack experiencing the issue before getting uploaded, as well as the log file.
... and if I may make a suggestion, don't delete files because you think they're not valid. Developers may include comments or extra data that works with 3rd party programs. Empty files may exist to be overridden/extended by addon datapacks.
This duplicates REALMS-1299. That error message is correct; the JSON is invalid according to the JSON spec. Strings are not allowed to contain literal newlines. GSON accepting it despite the spec is merely for convenience; if you want your pack to work on realms, you should comply with the spec.