From my testing, here is a (hopefully) comprehensive list of how Realms breaks datapacks on upload:

• All predicate files are deleted (new 1.15 feature)
• Empty function files are deleted (which causes errors if something relies on an empty function, like a function tag)
• Any file with "world" in its name has "world" replaced with the world's name.
• Other "unneeded" files that may be important are deleted, like readme, licence, or credits files (not game breaking but potentially detrimental).

Attached is a simple datapack that replicates these issues, along with it's realms-upload.log.