I can confirm the same behavior on my end, including with the raw Graph query, so the issue seems to be on the backend. The good news is that it does seems to work with $expand:
GET https://graph.microsoft.com/beta/security/cases/ediscoveryCases/bc4fb3eb-321f-44cd-a2e5-73ac1ece9404/searches/30ff976a-88b4-48a9-a321-da55af8cad41?$expand=*
Or via PowerShell:
Get-MgSecurityCaseEdiscoveryCaseSearch -EdiscoveryCaseId bc4fb3eb-321f-44cd-a2e5-73ac1ece9404 -EdiscoverySearchId 30ff976a-88b4-48a9-a321-da55af8cad41 -ExpandProperty LastEstimateStatisticsOperation | select -ExpandProperty LastEstimateStatisticsOperation