This is a great deep-dive on debugging a Java 21 virtual threads issue. Virtual threads have the same API as OS threads, so they're very easy to migrate to. Debugging a difference in behavior, on the other hand... netflixtechblog.com/java-21-virt...

Java 21 Virtual Threads - Dude, Where’s My Lock?

Getting real with virtual threads

this uh doesn't look great

This feels like the kind of bug you could really only catch in advance with a model checker. And, even then, you'd have to model things at the right granularity to actually catch it.