When running an A/B test, the analytics will still be associated with the primary index, which means all events will be associated with the primary index and not the replica. The replica index will appear to have no analytics when viewing the Search Analytics dashboard of the replica.
You can view the analytics for variant B (ie. the replica) of the A/B test by clicking through to the Analytics via the A/B testing dashboard using the analytics button to the right of each index's description. 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.