It looks like there's some constraint on having only one task targeted to a milestone at a time?
Also it looks like clicking the text gives you the popup but clicking the (+) icon gives you a new page? If true, I'd say that's almost certain a bug (maybe a different bug).
OK, so there's a couple of weird things here:
It looks like there's some constraint on having only one task targeted to a milestone at a time?
Also it looks like clicking the text gives you the popup but clicking the (+) icon gives you a new page? If true, I'd say that's almost certain a bug (maybe a different bug).