https://hibernate.org/orm/documentation/6.3/

From Migration guide

• Design changes to map HQL Numeric Literal Types to align with Jakarta Persistence 3.2 Specification changes.
• Internal design change for existing uninitialized proxies Batch Fetching and LockMode that improves batch performance in that uninitialized proxies with a different lockmode are not read.

Hibernate orm 6.3.0 release blog contains more details about the 6.3 release. https://in.relation.to/2023/08/31/orm-630/

Not as much detail is in the 6.3.1 release blog which does link to the jira system link https://hibernate.atlassian.net/browse/HHH-17221?jql=project%20%3D%20HHH%20AND%20fixVersion%20%3D%2032188