Different JDKs behave differently.
There are two major VM implementations, hotspot and OpenJ9 with a very different memory layout and garbage collection.
Hotspot is widely known and one distribution (Oracle OpenJDK) is the JDK used in jenkins. However, there are other hotspot distributors, e.g. AdoptOpenJDK Hotspot, Amazon Corretto, Zulu, Graal and liberica (and probably others).
There are also two main OpenJ9 distributors. IBM J9 (formerly closed source) which was open sourced as Eclipse OpenJ9. There is currently one binary distributor of the VM, which is AdoptOpenJDK.net again.
As different distributions and VM implementations may have different defaults, Shiro should be tested against multiple of those. For example, OpenJ9 will have "better NPEs" enabled by default, while most hotspot distributions will not. Also, the IllegalAccess Permissions are stricter.
The goal is to add additional travis jobs.