As far as I can tell it is currently not possible to change a dynamic URL once it is defined. This would be useful for example when the path to an image has been changed. Now the field has to be completely redefined.
thanks for reporting this, I will try to reproduce it and then create a bugreport to fix it.
this should be already implemented in 220.127.116.11 release of IJC. I am not sure if the information went back to you, as our internal linking between the issue and this forum topic was broken, so just to make sure you know.
Wishing all the best