Same problem when imported from iPhone 7 Plus / iOS 11.1.2 to MacBook Pro (2009) / OS X El Capitan 1011.6 (latest version possible for this Mac) / Photos 1.5 (370.42.0). Problem did not occur when imported to new iMac / OS X High Sierra 10.13.2 / Photos 3.0 (3251.12.190).
When I deleted the grey-line photos from my MacBook Pro / Photos and imported again, the photos imported without the grey line. Also, not all of the photos that transferred the first time had this problem. Not even all photos from the same series, week, or whatever. I would guess it was a transient transfer error.
I never use the auto-delete feature for just this possibility. I want to be sure my photos have transferred properly and that all of them have transferred before I delete them. It's a good idea to back up photos before deleting them, too.
My sister had years of photos of her young daughter on her computer (Windows PC) and never backed them up. One day, her hard drive crashed. Lost everything permanently. (Well, professional service said it might be able to recover the data for $7,000.) Better safe than sorry. Don't take the easy way out. Put in the marginal extra work and manually delete photos.
Note that as a software engineer (30 years, mostly systems programming, e.g., OS and/or embedded) I can say with great confidence that diagnosing a transient issue like this is a nightmare, because it typically is very difficult to reproduce. In this case, it could be a cable problem, either defective cable or just loose connection. It could be an OS problem, e.g., timing of asynchronous multitasking, on either end. It could be an applications problem on either end. It could be a memory problem on either end. And then there's the infinite list of possibilities that no one would think of until they're discovered.
I think the only solution here is to be diligent in managing your images. Even if everyone had this issue for the same reason and Apple fixed it, there's sure to be more issues in the future that diligence will help protect you against.
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