Workboard custom filter: Apply "Not in" to milestones

We have a project “Active Tasks” that has a workboard with columns “Backlog”, “Next”, “Now”, “In review”, “Done”.

The Tasks in this workboard belong to different but related projects project1, project2, … . We find that we have a better overview over all our tasks if we use only one workboard for all our tasks, hence “Active Tasks”.

Some of the project1, project2, … projects have milestones.

Sometimes we want to filter our main workboard by project. Filtering with a custom filter for “project1” shows all tasks in this project and also those in its milestones. Perfect!

Sometimes we want to check if there are tasks in our workboard that have no association with any of the project1, project2, … list. The custom filter offers a “Not in:” modifier for this purpose. However, “Not in: project1” still shows tasks assiciated with milestones of project1. How can we apply “Not in:” also to milestones?