In the old gradebook docs, "When using the lowest or highest score rule, an assignment group must include at least one score, plus the number of dropped scores and the number of assignments that should not be dropped. For instance, if you have a rule for dropping three scores and one Never Drop assignment, you would need five student scores in the assignment group to have all three lowest or highest scores dropped."
Is it different for new gradebook? We are getting some very strange activity in an assignment group that does not yet have the correct number of assignments to start dropping grades. I'd almost say it is random what is getting dropped.
I'm not sure when the rule was first applied to this assignment group, but it shouldn't make a difference.
Video with details - Did this on the fly, so not entirely professional. But like a good citizen it is captioned!