I don't think that's the issue, It's a simple bug Lightroom is importing using the Modified date, and not the created date.
I have this problem constantly, some pictures go into the correct folders, and others go into the wrong folders, it's clearly a case of some sort of bug, as I name my images by date and camera model as well as sorting by date, right now from my last input I have a photo called "2016-08-06 18-59-28 iPhone 6s IMG_2420.jpg" sitting in my 2017/December folder, looking at the properties with file explorer I see the file was "Created 21 January 2018" on my file system but modified "12/21/2017" but if you look at the Date captures in the Exif: 8/6/2016" which is the date the picture was taken, I think import is using the modified date, which might be dorked around with due to iTunes backups or some such thing.
I'm not really sure how to file the bug into Adobe, but I'm pretty sure this is the issue.