I've never used the card reader to import to the iPhone. I've seen it used for the iPad, however using it to import that large a group of photos could cause a problem. I've never even read here of others describing using that method here. I would wait to see if anyone else has used that method and could comment.
Okay, so you are assuming it is iOS 11 that caused the problem. What troubleshooting have you tried then? User troubleshooting consists of restart, restore from backup, restore to factory; testing after each step. After creating a current backup of the device, restore to factory and set the phone up as new. Attempt your import without adding any additional content to the device. If it fails, then I would suggest contacting Apple Support for additional assistance. Contact Apple Support
Please describe the method you are trying to import these photos with.
Stock Photo app using Apple sd card reader.
I have been using this method since iphone5 came out and regularly import>1000 photos at a time from a single card without any problem until the upgrade to iOS11
IOS11 photo import issue
