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.
Thank you for your report!
We're actually already tracking this issue in REALMS-1805, so I resolved and linked this ticket as a duplicate.
If you would like to add a vote and any extra information to the main ticket it would be appreciated.
If you haven't already, you might like to make use of the search feature in the future to see if the issue has already been reported.
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