This scenario illustrates how the PostgreSQL JDBC driver behaves when either using the socketTimeout connection property or calling java.sql.Statement.setQueryTimeout().

When the socketTimeout is reached, the JDBC driver closes the underlying connection; whereas the queryTimeout leaves the connection in an usable state.

This basically forbids setting socketTimeout when connection pooling is used in a liberal fashion, as the pool is not informed that the connection is closed.
With "liberal fashion" I mean (using e.g. the Tomcat JDBC pool, the default of Spring-Boot): you're not explicitly setting either testOnReturn or testOnBorrow along with an validationInterval less than the socketTimeout.