Currently, if you are trying to set a watcher from the fieldChange.to smart value which enclose the user AAID in [] brackets, it will fail with:

Unable to watch work items
INF-74: [712020:xxxxxxxxxxx]

The solution is to remove the [] brackets, but it's hard to notice the problem with the above audit log right away.

Since the action expects the user IDs to only have numbers and colons, any other character can be reported in the error message.