Logical Decoding Callbacks

I'm trying to write a logical decoding plugin and I'm seeing some
interesting behavior around the startup callback. When I use psql and the
built-in SQL functions (like pg_logical_slot_peek_changes) to use my module
I see the startup_cb get called. I have written my own streaming
replication client and for some reason when I use my client the startup_cb
does not get called, but I can still see the changes just fine. The reason
why the startup_cb is particularly important in my case, is I'm trying to
set two dynamic OIDs for PostGIS geometry and geography types. I've tried
setting them both as globals and as part of the output_plugin_private data.

Any idea what's wrong here? Did I miss something in the streaming
replication documentation?

Thanks in advance.

Xavier