When using the built-in build completion trigger in azure devops, the predefined variable Build.TriggeredBy.BuildId is set to the build id of the triggering build. The Trigger Build Task could pass the value of Build.BuildId as an additional parameter with the name Build.TriggeredBy.BuildId to the triggered build, thus avoiding confusion. Right now the following can be added as a "workaround" to the additional build parameters to achieve this:
{"Build.TriggeredBy.BuildId" : "$(Build.BuildId)" }