It is regression bug from
LPS-34678 which only backported the fixes for portal not plugins. The fix for plugins is not enough to work. It needs LPS-26321 which added the PluginContextListener as pluginContextListenerContent in updateWebXml() in BaseDeployer class. However, LPS-26321 has too many commits.
Shuyang suggests that the simple fix could be adding
"<listener-class>com.liferay.portal.kernel.servlet.PluginContextListener</listener-class>" in web.xml in plugins for 6.0.x.