After upgrading Hibernate from 5.2.16 to 5.3.3 we are encountering a NullPointerException in FetchStyleLoadPlanBuildingAssociationVisitationStrategy::adjustJoinFetchIfNeeded because the field lockMode is null:

if ( lockMode.greaterThan( LockMode.READ ) ) {
	return new FetchStrategy( fetchStrategy.getTiming(), FetchStyle.SELECT );
}

The entity is a single table inheritance entity with multiple implementations, and the exception occurs when lazy loading of the entity proxy is fired.

We found out by following the stacktrace that in LegacyBatchingEntityLoaderBuilder.LegacyBatchingEntityLoader the lockMode is set explicitly to null:

public LegacyBatchingEntityLoader(
		OuterJoinLoadable persister,
		int maxBatchSize,
		LockOptions lockOptions,
		SessionFactoryImplementor factory,
		LoadQueryInfluencers loadQueryInfluencers) {
	this( persister, maxBatchSize, null, lockOptions, factory, loadQueryInfluencers );
}

Presumably, instead of null the current value of lockOptions::getLockMode should be passed here.