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.