Sub-stream plays, main stream does not
The table says what is ideal. The recorder decides what is permitted. Many entry-level and hybrid recorders cap the resolution an IP channel may use, often below a modern PC screen. Because the main stream defaults to the full screen, this is exactly the symptom you get.
Fix — set Main stream width = 1280 (gives 1280 × 720; height follows the aspect ratio automatically). This sits below almost every ceiling in use, and a till or dashboard stays entirely legible.
Then delete the channel on the recorder and add it again. A recorder reads a camera's capabilities once, when the channel is created. Leave the channel in place and it keeps the old figures — this is the usual reason a change appears to do nothing.
To find a recorder's real ceiling: try 1280; if it plays, step up to 1600 then 1920. If 1280 fails, step down to 1024 then 960. Re-add the channel each time; the first width that fails is the limit. If resolution is not the cause, hold the width at 1280 and drop the bitrate to 1024 — some recorders stop at bitrate instead. The codec is never the cause: both streams come from the same encoder and differ only in size.