So it seems this is the same issue as reported here
When i force GP to run using Rosetta, the BPM sync works again as expected.
I simply was not aware, that AU plugins will use the Compatibility mode by default, without me asking them to.
The last update for the BIAS Plugins has been Mid 2021, so i’m not very confident that they’ll ever provide a apple silicon compatibile version, so i worry i’ll be stuck on Rosetta forever…
Interestingly the BPM Sync works the first time (when i create the plugin) and Mainstage can Sync the BPM correctly despite running natively and using Rosetta for the plugins, so it seems there is a way to get this working in that configuration.
For now i contacted the PositiveGrid support to ask if they are planning to update the compatibility for the plugins, and the workaround with running GP under Rosetta is sufficient for the time beeing.