I'm not really sure how this one has passed so long without causing more issues....
When a user is created in a remote application, this is passed to converge via the convergeAddMember() method in converge_master/apis/server_functions.php. This in turn calls out to all products with a ipConverge.onRegister callback.
IPB does not define this method, so the user is not added to IPB when a user initially registers in a remote application. As such, the request is sent to a queue (which is never processed) and the user never exists in IPB until the first time they login.
If I change the request from onRegister to convergeLogIn, the user is successfully added, although this is not exactly necessarily desired functionality (I might be adding a user from the ACP of another application, where I am not the user that is being added to IPB, for instance). The proper solution is to include an onRegister callback for IP.Board properly.