The invalid squiggle line is shown when you first check for isValid but no circle-cross which has the reason shown. The second time you check for invalid, the circle-cross is then shown.
It seems to me the show/no show state is initiall wrong and does not show on the 1st time.
I've found that when you have a validatable field with no entry, and then call isValid on the FormPanel, the notification circle shows, but subsequent calls of isValid makes the circle move in???
Something is happening that makes the circle behave different on 1st vs 2nd (and following) calls to isValid for a field that has no initial data.