This usually happens when the VCF file is edited or split using basic text editors or drag-copy methods. vCard files are sensitive in structure; even a small missing line break, encoding change, or broken contact block can make some entries unreadable.
I’ve seen this mostly when users try to manually cut one big .vcf into multiple files. The contact blocks don’t always stay intact, so Outlook or phone apps fail to read them properly.
A safer way is to avoid manual editing completely and split the file in bulk using a proper parser-based method. Something like GainTools Address Book Manager can load the full VCF and split each contact automatically without touching the internal structure.
One thing I’d suggest: always keep a backup of the original VCF before doing any split, because once corruption happens, recovery is messy and time-consuming.
