Write to Custom Session Properties

How are you doing this? If this is a Script Action and then a Navigation Action, there is no guarantee of the execution order. If this is a single script which writes the value and then uses system.perspective.navigate() it should be working, and I would need to actually see the script to help you out.

Do you see any errors in the gateway logs when you trigger this ActionPerformed Event?

No, (Custom) Session properties behave just like any other property. In newer versions of Perspective/Ignition system-use session properties (most properties in session.props) will ignore write attempts.