Stumped on this one. In Grails it seems one cannot define a default sort on multiple columns in domain mapping a la static mapping = { sort \'prop1 desc, prop2 asc\' }, or
Thanks to the link from GreenGiant, we see that the issue is closed as fixed as of version 2.3.
There is also example code:
static mapping = { sort([lastname:'asc', name:'asc']) }
It is working for me in 2.4.3