Sorry that I was not able to create a test case for this issue but I'm pretty sure that there is an issue introduced in Hibernate 5.2.2 related to connection release when an SQLGrammarException is thrown.
Steps to reproduce:
1. Take a Spring application using JPA2 and hickaricp/c3p0 connection pool
2. Trigger an SQLGrammarException by messing up the schema
3. Each failed JPA query will leak one connection eventually starving the pool

I attached the log that shows how c3p0 detects the leak (the same as hickaricp). For now I will stick to version 5.2.1.Final I hope that this issue will be fixed, sorry but I can't spend more time working on hibernate to create a proper test case, I need it to "just work".