In 5.0.x the default mode for JdbcResourceLocalTransaction was indeed ON_CLOSE, however for JtaTransaction it was AFTER_STATEMENT. It looks like this wasn't taken into account when the 'revert' to the default mode was added.
This is essentially a known issue: HibernateJpaVendorAdapter.setPrepareConnection mentions the enforced DELAYED_ACQUISITION_AND_HOLD mode on Hibernate 5.2 already. Admittedly, this should more explicitly mention JTA setups and WebLogic in particular... and have a dedicated note in the reference documentation as well.
The underlying technical problem is that we can't influence Hibernate's connection release mode depending on the transaction setup: simply because JPA doesn't allow us to reliably introspect the transaction setup in general. As a consequence, HibernateJpaVendorAdapter leans towards optimizing our common case: Spring-driven resource transactions with full control over the lifecycle of the underlying connection, which requires ON_CLOSE connection mode semantics. Hibernate has been changing the default a few times over the years, and our general policy is to enforce ON_CLOSE by default whenever we need to.
The easiest way out would be to set HibernateJpaVendorAdapter's "prepareConnection" flag to false, or to not use HibernateJpaVendorAdapter in a WebLogic JTA setup to begin with. Of course you may also simply override the corresponding Hibernate property, as you suggested.