In the Changelog for Snapshot 17w49b you can read: "When overriding a tag, you now append instead of replacing"
I have two Datapacks installed in my world that each have a tick.json with the filepath: data/minecraft/tags/functions/tick.json
If both datapacks are enabled, only one of them is working. I have to disable the working one to have the other working. This indicates that one overwrites the other. I prepared two test-functions that show the problems (a bit obnoxious):
I have 2 test functions that should spam "Function" and "Function1" into the chat, but only "Function1" is visible in the chat.