Reading the documentation here:
"The SI server uses identityserver-contrib-membership. This project allows the ASP.NET 2.0 Membership Database to be used as the Identity Server User Store in IdentityServer4"
"You configure the connection string to the Membership database with the Sitecore:IdentityServer:SitecoreMembershipOptions:ConnectionString setting"
I get the impression that the Identity server can use user information from any domain stored in the core database, but it does not actually use the ASP.NET 2.0 Membership Provider, and will not use any custom membership providers (configured in web.config/membership element and domain.config)