When attempting to use any Non [a-z0-9_.-] character in any of many places for a singular advancement causes ALL advancements to not load (even all advancements from all other datapacks). Additionally there is never any error message indicating that something went wrong on either the client or server. An example root advancement that causes this issue has been attached.
(Note: before 19w11a you could get an error message to appear if you loaded a world that had such an advancement loaded - the server would crash and the game output would tell you what went wrong — the attached image and crash report show how the game responded before 19w11a)

If the Non [a-z0-9_.-] character appears in one of these then the error occurs (not necessarily an exhaustive list):

• Trigger values
• Item names (in both icon and criteria)
• Background value
• Reward function names

If the character appears in one of these then the error does not occur:

• Text components (ex: title and description)
• Key names
• Non-string values
• Value of "frame"

If a value is invalid but does not contain a Non [a-z0-9_.-] character then an error message is given describing the issue.

The below text from the server log demonstrates the fact that no advancements at all load.

Beginning with only vanilla loaded:

16.05 14:27:53 [Server] INFO [14:27:53] [Server thread/INFO]: [Addgame: Reloading!]
16.05 14:27:53 [Server] INFO [14:27:53] [Server thread/INFO]: Reloading ResourceManager: Default
16.05 14:27:53 [Server] INFO [14:27:53] [Server thread/INFO]: Loaded 523 recipes
16.05 14:27:53 [Server] INFO [14:27:53] [Server thread/INFO]: Loaded 570 advancements

Above is the expected result. Note the number of advancements loaded appears.

16.05 14:28:05 [Server] INFO [14:28:05] [Server thread/INFO]: [Addgame: Enabled data pack [file/ioe]]
16.05 14:28:05 [Server] INFO [14:28:05] [Server thread/INFO]: Reloading ResourceManager: Default, ioe
16.05 14:28:05 [Server] INFO [14:28:05] [Server thread/INFO]: Loaded 523 recipes
16.05 14:28:05 [Server] INFO [14:28:05] [Server thread/INFO]: Loaded 17 custom command functions

Above is after enabling the data pack with the attached advancement. It stops loading data without an error message and does not load any advancements (as can be seen by the lack of message saying how many advancement were loaded).

16.05 14:28:14 [Server] INFO [14:28:14] [Server thread/INFO]: [Addgame: Disabled data pack [file/ioe]]
16.05 14:28:14 [Server] INFO [14:28:14] [Server thread/INFO]: Reloading ResourceManager: Default
16.05 14:28:14 [Server] INFO [14:28:14] [Server thread/INFO]: Loaded 523 recipes
16.05 14:28:14 [Server] INFO [14:28:14] [Server thread/INFO]: Loaded 570 advancements

Once again the expected result is given after disabling the data pack.