This is a current limitation of the debug hover that it does not have a full language understanding and due to that tries to find the hovered expression in the variables view. If the hovered expression is not present in the variables view or has duplicates the hover will not show anything.

To improve this the debug hover would need to have an AST understanding of the language which is a feature request and not yet on the plan.

An alternative is that a debug adapter extension can choose to evaluate expressions on hover but this has the downside of potentially mutating the state of the program without the user being aware. Currently node debug is not doing this.