My guess is that the 8mm app encodes the video slightly differently; iMovie was made just to edit standard iPhone/iPad video. I read a lot of complaints here about people wanting other cameras footage to be editied.
Your best bet might be to contact the 8mm developer and ask them if it is encoded as standard iPhone video.
But can you add the 8mm look after you have edited; or can that program only shoot the format?
I've shyed away from the 8mm apps as most shoot the footage with the effect on, so you can't go back and see it "normal". I want one that can encode the look later.
Or you could get the edit program "splice" and see if it can edit the video; it's quite a good editor in itself (try the free version it's fine for me).