Store Output Values tab missing from flow builder

I am trying to create a Flow using Flow Builder that invokes an Apex Action. My Action returns a wrapper with some InvocableVariables.

As per this help document on Flows I should get a tab to save the output values from my Apex InvocableAction in some variables. But this tab is completely missing.

For me it looks like this:

Any pointers will be helpful.

In tutorial sites I see it should be looking like this:

Adding more detail:

After hitting this endpoint from REST explorer of Workbench, I can see the inputs and outputs arguments:

/services/data/v48.0/actions/custom/flow/myflowwithapex

However, my wrapper output from apex (consisting of global variables annotated with @InvocableVariable) are NOT displayed.