When running an A/B test, the analytics will still be associated with the primary (control) index. This is because the requests sent from your frontend are initially sent to the same (control) index to mitigate the need to modify any of your code. When the engine receives the request, it is then routed to one of the A/B test variants.
This means all events are associated with the primary index and not the replica and therefore the replica index will appear to have no analytics when viewing the Search Analytics dashboard of the replica.
You can view the analytics for other variants of an A/B test by clicking through to the Analytics via the A/B testing dashboard using the "View Analytic" dropdown button to the right of Metric Breakdown section. More info on this here.
This opens a new tab with search analytics for the primary index but filtered by an analyticsTags for variant in the format: alg#abtest:${abTestID}+variant:${variant}
These analyticsTags allow you to view detailed analytics for each variant.