in a project from which I export three audio tracks (STEM export):
how do I export the Master automations?
So that when I import the stems into a new project, the master automation also appears at the top.
[Ardour 8.1 on Linux]
Stem export is just audio data - either raw unprocessed, or with plugin FX applied.
It does not include any meta-data nor any plugin settings or automation.
The idea for stem export is to use a common format that any audio-software supports (here: .wav files)
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What you could do: Copy the project with the automation (Menu > Session > Save as), exclude all media, and delete any unused tracks, only keeping the master-bus.
Then import the stem-exported audio files into that session.