Overwrite to Primary Storyline weird behavior

My colleague showed me a strange occurrence he's been seeing occasionally in FCP. He has two clips in the timeline, side by side with no gap between them. Clip 1 is connected and clip 2 is in the primary storyline. When he overwrites clip 1 to the PS (using the keyboard shortcut Option-Command-Down Arrow) it inserts a frame of black after the clip and pushes the rest of his timeline down.

The only thing I can think might be causing this is that he's using 23.98 media in a 29.97 timeline, but I've never seen this behavior before.

Hey Bill,

Thank you for the information.

I will do some tests later, but please use the feedback form to report this to Apple.

- Ronny