In Lilliput PR#23 (https://github.com/openjdk/lilliput/pull/28), I introduced some code that forces deflation of monitors on objects that are about to die. Discussions around a possible upstreaming of this code (https://github.com/openjdk/jdk/pull/13721) comes to the conclusion that this should not be necessary, and indeed, I could not reproduce the original problem that I was having in Lilliput with the code removed. I assume some changes in upstream and/or Lilliput since then have fixed the root cause.