This problem is usually caused if you use the Pixel’s transfer feature before using SMS Backup & Restore’s transfer feature. The Pixel’s transfer feature tends to create only the text based parts of MMS messages but does not transfer the media, leaving a skeleton behind. SMS Backup & Restore’s transfer then detects those existing messages as duplicates and does not restore the remaining parts of those MMS messages.
To fix this problem, you’ll need to delete the existing MMS messages from the phone and then perform another transfer/restore using SMS Backup & Restore.
You can use the “Delete messages or phone calls” feature in SMS Backup & Restore to easily delete all messages.