xpack.security.sessionTimeout has been renamed to xpack.security.session.idleTimeout, so it's not necessary to configure both. I'd recommend just keeping xpack.security.session.idleTimeout. You can also make that setting more readable by specifying it as a duration like you've done for the lifespan:
xpack.security.session.idleTimeout: 15m # timeout after 15 minutes of inactivity
None of that is the cause of your problem though. Can you double check that you have xpack.security.encryptionKey set on BOTH of your Kibana instances, and that this value is identical? Having different encryption keys set on different instances is a common cause of this problem.
If that's already configured properly, then we will need debug logs to triage further. Set the following, restart Kibana, and capture the logs which show the session timeouts happening:
In addition to what Larry said, make sure that ALL session related settings (xpack.security.session.*) are identical for all Kibana instances you have.
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