We have numerous closed bugs relating to this assertion failure but it seems to have reappeared:

2021-06-14T01:15:13,296Z] [630.006s][warning][gc] GC locker is held; pre-dump GC was skipped
[2021-06-14T01:15:13,296Z] #
[2021-06-14T01:15:13,296Z] # A fatal error has been detected by the Java Runtime Environment:
[2021-06-14T01:15:13,296Z] #
[2021-06-14T01:15:13,296Z] # Internal Error (t:\workspace\open\src\hotspot\share\oops/compressedOops.inline.hpp:133), pid=5236, tid=4568
[2021-06-14T01:15:13,296Z] # assert(!is_null(v)) failed: narrow klass value can never be zero
[2021-06-14T01:15:13,296Z] #
[2021-06-14T01:15:13,296Z] # JRE version: Java(TM) SE Runtime Environment (18.0+2) (fastdebug build 18-ea+2-13)
[2021-06-14T01:15:13,296Z] # Java VM: Java HotSpot(TM) 64-Bit Server VM (fastdebug 18-ea+2-13, mixed mode, sharing, tiered, compressed class ptrs, z gc, windows-amd64)
[2021-06-14T01:15:13,296Z] # Core dump will be written. Default location: T:\testoutput\test-support\jtreg_closed_test_hotspot_jtreg_applications_kitchensink_Kitchensink_java\scratch\0\hs_err_pid5236.mdmp

Unfortunately due to a full disk we have no test artifacts related to this failure.