The class file API verifier seems to be based off a version of hotspot verifier before Mar 3 2021. We are thus missing a few patches in the hotspot verifier:
JDK-8350029 JDK-8340110 JDK-8330606 JDK-8314295 JDK-8270398 JDK-8267118 JDK-8262368 JDK-8272096 JDK-8349923

Synchronization allows the ClassFile API verifier to catch up with upcoming verification changes for Valhalla strict fields as well.