Undo behaves differently in Cubase Pro 10.5.
Within the Key Editor Undo used to work exclusively on the part that was active inside the Key Editor. Now it Does not.
As you can see in the first video Cubase 10 Undoes edits made to the Flute part. Regardless of where in the history they were made.
In the second video you can see that Cubase 10.5 undoes edits made to the English Horn part. Even though the Flute part is currently active in the editor.
There is nothing in the manual about this behaviour.