As you indicate, it's normal behaviour for RTs to be regenerated whenever a song is reopened in BIAB.

If you wish to stop this, simply freeze the tracks you want to keep. On the image below, I've highlighted the 'Freeze' icon present on the mixer in BIAB (Windows). I anticipate that BIAB (Mac) will have something similar.

While the track still needs generating after freezing, when the file is reopened, it will always generate the same as it was at the point of freezng.

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Audiophile BIAB 2025