This article describes an issue with setting a projection using SearchClient.Instance.Conventions.UnifiedSearchRegistry.

The code is examplified below

SearchClient.Instance.Conventions.UnifiedSearchRegistry
.ForInstanceOf()
.ProjectTitleFrom(x => string.Format("{0} [{1}]",x.SearchTitle(), x.SearchFileExtension()));

 For more samples please see here.

The issue is identified in FIND-2625 and resolved in Episerver.Find 12.7.1 which can be found here.  The recommendation is to update the Nuget package to resolve this issue.