This is the same issue seen in #5885, but now extends to OpenJDK 8 as well as OpenJDK 12 (these are all I've tested on) under > 9.2.10.

Environment Information
JRuby 9.2.10.0, 9.2.11.1 invoked with --dev --debug

x86_64, macOS 10.15.4 (Darwin Kernel Version 19.4.0)

Expected
Calling the setInitialMode on net.sf.saxon.s9api.Xslt30Transformer (which is defined on the abstract class net.sf.saxon.s9api.AbstractXsltTransformer) should not explode with a NoMethodError

Actual
On 9.2.8, 9.2.10, and 9.2.11 under OpenJDK 12 this goes boom. On 9.2.8 under OpenJDK 8, it doesn't explode and works as expected (as reported in #5585). On 9.2.10 and 9.2.11 under OpenJDK 8 it fails in the same way as it does under OpenJDK 12.

I'll add a test case and see if I can reproduce under master too.