I assume it's a cardinality of n:m between CI & Incident?
Because one CI can theoretically have n active Incidents and one Incident can have m active CIs.
If it is I would rather go for a field on CI level like "hasActiveIncidents".
Add a business rule to populate it, e.g.
$(ChildFold("CI#", RecId, "Incident#.CIAssociatedActiveIncident", "1", "+"))
Since your relationship is filtered this should be fine.
Could it be because you are not using the Internal Reference name of the relationship (listed just below the display name in the relationship definition)? I thought that is what is expected after the period. In my tenants, that relationship has an internal reference name of Active. So the correct expression might be
$([Incident#.Active]RecId != "")
However when used, you get the warning, "The relationship [Incident#.Active] is 0-1 to many or 1 to many. Selection will choose a single object from among the ones available.", which I believe what Florian was alluding to.