That’s the key point here, only it’s a bit more subtle than even that.
I can reproduce this on Plasma dragging and dropping video from Dolphin - however it looks like it’s an issue with the bin jobs running on the clip and identifying its contents. Because if you drag and drop the same clip again after the bin jobs have completed, it adds both audio and video to the timeline, the same as if you drag it from the bin.
Please open a bug report for this one - you can link to the discussion here in it.