Hmm, not sure what is going on here. It appears the priority is really a number, but that doesn’t work in the binding either.
1 = Diagnostic
2 = Low
3 = Medium
4 = High
5 = Critical
The only thing way i could do what you wanted with two bindings, on a label i created a custom binding to the priority with no transform. On the text property i created the transform you have, which worked but i want to say you shouldn’t have to do this.
I “THINK” a regular tag binding doesn’t have access to that level. This is why the expression worked. But I’m really not sure someone else would have to confirm