404 only when RelationManager enabled (tenant setup, policies applied)

Company (tenant, route by slug)
CompanyRequestEvent → belongsTo(Company)
CompanyRequestEventSector → belongsTo(CompanyRequestEvent) + hasOneThrough to Company

CompanyRequestEventResource: protected static ?string $tenantOwnershipRelationshipName = 'company';
CompanyRequestEventSectorsRelationManager: protected static string $relationship = 'companyRequestEventSectors'; + same tenantOwnershipRelationshipName

Event edit page works without RelationManager
Enabling RelationManager → 404
Tinker confirms $sector->company and $event->company return valid models

Policies for all 3 models with all methods returning true
AuthServiceProvider registered
hasOneThrough works in Tinker
TenantOwnershipRelationshipName set correctly

Has anyone seen 404 only when RelationManager is active, even with policies and tenant ownership set?
Where in Filament v4 would you debug authorization / tenant checks for RelationManagers?