Could you please try below context object.
${notification.message.author.login}
On a default immediate_general email template this context object is being used.
#set ( $eventUser = "${notification.message.author.login}")
#set ( $eventUserRanking = "${notification.message.author.ranking.name}")
#if ($notification.moved)
#if ($notification.merged)
#set ($eventText = "merged a")
#else
#set ($eventText = "moved a")
#end
#else ttadej- As you have tried all the possible context object which shown below and no other context object is mentioned on the documentation.
$actorLogin
$notification.message.actor.login
$notification.actor.login
$notification.message.lastEditUser.login
Open a ticket to lithium support for this. Or if it is not available post an idea here.
For others running into the same issue, here is the response from Lithium:
"Our product team agrees that this is a bug and I've opened an investigation to determine a possible fix. I'll keep you updated on our progress."
