According to the description of “New member on card (instant)” it should “Emit new event for each card joined by the authenticated Trello user.” In reality, workflows triggered this way are triggered anytime a new member is assigned to a card within the chosen board.
This can be worked around by adding a filter, which checks for the last member id in steps.trigger.event.membersId, but that requires the hardcoding of the authenticated member id, since the (authenticated member) id is not returned with the trigger metadata.
Even with the work-around, however, the workflow trigger log quickly becomes cluttered with dummy calls, which cut off immediately within their first step. I think we should add a check for the member ID within the trigger as per the description.
Thank you so much for pointing me in the right direction. I’ve now created a github issue. I actually looked through the repository before writing here, so I already have an idea on how one can fix the bug - I’m planning on getting around to it on the weekend, if no one gets to the issue first.