I’m using “Merge” as midi recording mode. If I click on a part, getting into the editing window and record some new stuff there, it creates new parts instead of merging the newly recorded notes into the existing part. Frustrating. Why is this?
That’s because you have new parts selected in the transport / midi recording mode.
Press F2 for the transport. Select Merge on the left side of the transport. Page 250 of the manual.