Hello,
Thanks to the post in German, “Poor control behaviour or overshoot on the ESS”, I found a solution that works.
https://community.victronenergy.com/uploads/short-url/1yGMSZUMRc0yyHJiYgkQ5rkwZPm.pdf
The issue was a poor connection at the Multiplus current-measurement jack socket.
After plugging and unplugging the jack (and applying a little contact cleaner), the ESS system works normally with the mains connected to ACIN.
For now, I have stayed on Venus 3.14, but I will update it to 3.75 and keep you posted.