After exhaustive investigation we realized that when a WAR uses a shared library in Weblogic, EJBs and many other object instances are not being garbage-collected, even after the undeployment and deletion of the war.

weblogic.xml:

<library-ref>
{{ <library-name>SSPCommonLibsWar</library-name>
</library-ref>}}

inside common libs war :
{{hibernate-core-5.2.11.Final.jar
}}
We are absolutely sure that no other library inside SSPCommonLibsWar causes this because we exclude them one-by-one.
Only when hibernate-core-5.2.11.Final.jar existed inside SSPCommonLibsWar the memory leak happens.