Hi, Jeff!
jmix.ui.http-session-expiration-timeout-sec setting is not using now.
You should use standart spring boot property server.servlet.session.timeout as it described here Servlet Web Applications :: Spring Boot and you should point out Vaadin parameter vaadin.servlet.close-idle-sessions with true value(default value is false).
The behavior of sessions expiring in Vaadin applications is describied here How to manage the Vaadin application lifecycle.
We will discuss of adding more concrete description of it in our documentation.
Best regards!