Not sure if this is a bug or already being reported. See if you can reproduce it.
I tried this on a brand new setup. This doesnt work on nopcommerce demo site as it is blocked.
STEP 1. Select 2 products with attribute.
Apple MacBook Pro 13-inch
Asus N551JK-XO076H Laptop
STEP 2. Export it.
STEP 3. Import it. Error.
1. products with NO specification attribute
2. export/import single product with attribute.
Let me know if you guys having this problem. Please share if you found a way to bypass this..