Hello!
I have had an unbelievable struggle getting this automation to do it's thing, and I'm very close to the finish line, hopefully someone can point out the silly mistake I've made and get this across the finish line.
Here's the layout as is.

The objective to give people the ability to flag a particular user in a comment by putting an exclamation mark after their mention. The automation should then pick up that user's ID string and add it to a custom multi-user picker.

Using a conditional check of {{comment.body}} for \[~accountid:.+\].*! works perfectly, and the below smart value (and string it outputs) seems like the right combination to put into the multi-user picker, but no matter what it gives me the below error in the log.

The second string in the log is a .asJsonArray render from the {{Test}} custom smart value, which has the same smart value is as in the JSON additional field - just with ".asJsonArray" at the end. I've attempted to have the two strings with no space between the comma and second string, only the comma, only a space, asJsonString, asJsonArray - no luck with any of them.
So after all this : what on earth is the delimiter or format needed for multiple id strings for them to to be accepted by a multi-user picker box?