There are two different parts that can modify the "subseconds", TimeSpanMillisecondsPart (milliseconds) and TimeSpanTicksPart (ticks). The milliseconds is limited to 0 to 999, while the ticks has more accuracy (up to 7 digits).
The 'g'/'G' formats use ticks by default. So when you select the "Milliseconds" in the listbox, you are saying that only the milliseconds part should be editable. Since it's not present in the TimeSpanEditBox (based on the current format), all the parts are disabled.
You can use a custom format like "+d.h:m:s.iii" to use the milliseconds parts, by setting the TimeSpanEditBox.Format property.
The format specifiers are modelled after the DateTime type. The TimeSpan did not support formatting when we developed the TimeSpanEditbox. This was introduced in .NET 4.0, but we have not changed our format specifiers to match the .NET 4.0 versions at this time.