Can I use USER fields to define the ""Which records?"" clause in an Automation

I have a table called USERS. 
The key is This_User (which is type 'User').  There is one record for each user.
I've added a text field with logging there called 'Updates Log'

I have another table called WORK.  It has lots of fields including a formula text field called 'Work_Name'

I have an automation as follows:

• Trigger = whenever any field in WORK is modified
• Action = Modify USERS where This_User = "Triggered: WORK | Last Modified By"
• The info in this modified record(s) will be...
       Updates Log = "Triggered: WORK | Work_Name"

When I update any field in the WORK table, the automation trigger shows as complete and the action shows as complete, but the 'Updates_Log' field is not updated.

Thanks!