Network join performance variation between XBee SX modules

I’m using XB9X-DMUS-001 modules (XBee SX) and notice that some modules join the network very reliably upon power up while others require network scan or discovery messages.

As I power up modules as routers and configure them for API mode, most modules automatically join other stand alone routers (no coordinator) or join coordinator and start receiving and responding to RF messages.

Some modules however, will consistently not join the network with other routers or even coordinator until a network discovery is done, ie ND, NT messages to the network as a broadcast. In addition, some response to their Node information has to be replied to then it seems they can communicate to broadcast to all messages.

Is this typical behaviour and what dictates why some modules join very quickly and consistently just on power up while others require some level of interaction?

No that is not normal. I would suggest making sure that the modules are at default values. If they are, then you should contact Digi Technical Support either via email or by submitting a case.