The fact that it reverts to A1 is intentional - Nitro only exposes one preset slot to the host, so whenever you open the session it will be A1 that is active. However, the settings of the preset of your choice (the active one on session save) should be restored in slot A1, so you should get the same sound. Are you able to identify the parameters that change their values over save/load? Or maybe everything is different?
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Hubert Pietrzykowski
DSP Engineer
PSPaudioware.com