Texts messages
Recently I started receiving group texts from my son on my iPad. If he sends a text to me alone I receive it on my iPhone.
iPad (10th gen) WiFi
Question marked as
Check Settings > Messages > Send & Receive on both devices. They are likely set differently.
Another possibility is your son uses different addresses in the group vs. individual message (e.g., one uses your Apple ID while the other uses your iPhone telephone number).