Ok, I conducted some more Tests.
I started with an completely unmodified TDBAdvGrid.
Then added goEditing to the grid Options to enable the inplace edit.
Result: no problematic behaviour.
Then I looked into how our grids are usually set up. I was able to narrow it down to one property: EditPostMode
The default is epCell. If you change that to epRow, the problem occurs.
To sumarize, steps to reproduce the issue:
1. Take a TDBAdvGrid
2. Change EditPostMode to epRow
3. Add goEditing to Options
I've honestly no idea what EditPostMode does, and how epRow and epCell differ from each other.