Currently there is a workaround required to edit exported emf files (e.g., minimum spanning trees) inside of PowerPoint:

• Step 1: Insert the emf file as graphic in PowerPoint (e.g., by drag and drop)
• Step 2: Copy the inserted graphic with Ctrl + C
• Step 3: Choose in menu "Edit > Paste Special" (German: "Einfügen > Inhalte Einfügen")
• Step 4: Choose "Windows Metafile"

This should give you a copy of the graphic that can be ungrouped and edited.