I've tried:
Value: expr:@{Document["uid:uid"]} in a Execution Context > Set Context variable (traindocuid)
and get a blank(null?) when I use the traindocid variable in a User Interface > Add Info message as a check.
Message value is: traindocuid = @{traindocuid}. For the output, I get "traindocuid = " then nothing.
No errors are generated on the screen or in the logs.
As a sanity check I substituted: expr:@{Document["dc:title"]} in the Set Context Variable and this works as expected.
Am I missing something?