Monitor synchronization issue in JRuby 9.2.14.0 #6526

Environment

• JRuby version: jruby 9.2.14.0 (2.5.7) 2020-12-08 ebe64ba
• Operating system and platform: Linux 4.4.0-1118-aws Corrected a few typos #132-Ubuntu
• With Sinatra, AR 6.0.3, Redis 4.2.5, Concurrent Ruby 1.1.7

Expected Behavior

Service should run similarly to how it did in JRuby 9.2.6.0

Actual Behavior

Under high load, we are seeing threads appear to lock up. They are all within Redis#synchronize blocks. In extensive conversation in Matrix, @headius believes he traced this to a bug in CRuby's Monitor implementation. See the stack trace for full details:
jstack.log

I initially thought that this was a reoccurrence of #6326 but it appears to be a separate issue.