Setting modalTransparencyBlur to 0 gives blur when Alert ist shown on top of modal Dialog

Having opened a TitleWindow with modalTransparencyBlur: 0; and opening an Alert (with default modalTransparencyBlur) shows the following behaviour. The Alert leads to blur, closing the Alert removes blur, closing the TitleWindow shows blur again.
Seems to me, that PopUpManager.removePopUp() fails to set the blur correctly.