I tried to map semicolon ";" to a key (left the key left to [1] on the top row).
The XML file included ";" as output definition.
But when I installed the bundle, the semicolon was not possible to enter, instead (if I remember well) it turned into "|".
I tried to enter escaped form of semicolon ";" (the ";" would probably work too) and it fixed the layout. I did the change from Ukelele editor or manually.
Next time I opened the layout file, I was surprised that it showed the ";" again and not the escaped form. I suspect Ukelele to serialize it as plain ";" what prevents proper function for the key.
Used Ukelele Version 3.5.12 (396)