In our environment (Marketo/MS Dynamics CRM), we've always needed two separate links: one for the Marketo lead record; another for the MSD lead/contact record:
What we are noticing more of lately is the MSD link is invalid since it doesn't include the GUID - meaning the alert is being sent before the bi-directional sync is taking a bit longer than it used to. So we had to increase the wait step in our campaigns (from 20-30 minutes):
Oddly, everything seems to be running slower these days: the bi-directional sync (CRM contacts take upwards of 45 minutes), trigger campaigns (upwards of 30 minutes even though there's no backlog in the campaign queue, etc.)
Thanks a lot Dan! Helping us get to the bottom of this.
glad you found the solution! make sure to mark the question as answered so others don't have to review it.