We don't yet make full use of this Koha::IssuingRule in koha, however I have now found that the get_effective_... method within it behaves differently to the some of the C4 modules which query the same table.. In C4::Circulation there is a hard coded default should the default rule not be set (*, *, *) which claims a loan of 0 days. This default either needs to be coded into get_effective_issuing_rule or a Koha::Exception thrown is such a rule is not found or something....
A library in Sweden that upgraded to 19.05.x says that circ rules involving "all" patron categories and/or itemtypes stopped working after the upgrade. Only rules with specific itemtypes and patrone categories work. Could this be related to the problem described here?