Collation with upper and numeric comparing in unexpected way

I am using Postgres 17 and trying to configure a collation that sorts upper case before lower case and includes numeric sorting:

CREATE COLLATION testsort (provider = icu, locale = 'und-u-kf-upper-kn’);

These comparisons are working as I expected:

SELECT 'id-45' < 'id-123' COLLATE testsort; -- true (45 before 123)

SELECT 'id' < 'ID' COLLATE testsort; -- false (upper case before lower case)

However combining them resulted in an unexpected result:

SELECT 'id-45' < 'ID-123' COLLATE testsort; -- true

I thought that last one would be false because “ID” would come before “id”. Is there a way to configure the collation to achieve that? I’m trying to match the sorting behaviour in external application code.

Thanks for any help,
Matt