".partial" WAL file missing while promoting database in streamingreplication.

I use streaming replication in async mode. When master gets down, slave will be promoted using a trigger file. During this process, a ".partial" file will be created and a WAL file with the same ID will be created in a different timeline in slave (to be promoted as master). When master comes back as slave, it will be synced to the current master. This is the normal procedure as far as I understood.

But in some cases, ".partial" file is not getting created and peer DB which comes back as slave is unable to sync whenever this happens.. So, basically databases go out of sync when the ".partial" file is not created . Please suggest if this happens in any scenario and how to overcome this.