Commercial embedded vendors with RTOS capabilities,

https://www.ptc.com/en/products/developer-tools/perc

https://www.aicas.com/products-services/jamaicavm/

JikesRVM (https://www.jikesrvm.org) no longer active, but it was its own thing.

Then you have Azul Zing and OpenJ9, which use parts of OpenJDK for the standard library part, but the foundations are their own thing.

https://www.azul.com/products/prime/

https://developer.ibm.com/components/open-j9/

Then you have that mobile OS, which isn't really JVM compliant implementation, but it is close enough for many workloads and libraries to work on top of it.