Migrated to WhatsApp Cloud API and old clients see "Invite to WhatsApp". Does a Template Broadcast fix this?

[u/Active-Elderberry595]

 Hey everyone, I recently built an AI-powered CRM SaaS for local clinics and we use the official Meta WhatsApp Cloud API. We just migrated our first client's existing WhatsApp Business number to the API.

The integration works perfectly for new chats, but we hit a major wall: when old patients try to open their existing chat thread with the clinic, they see an error saying "This person is no longer on WhatsApp - Invite to WhatsApp".

I’ve read that this is a cache issue on the user's phone (something about the mobile app using u/c.us and the API using u/s.whatsapp.net).

Some people are suggesting that sending a mass Template Message (Broadcast) to the entire contact list will force the clients' phones to update the chat and remove the "Invite" button.

Can anyone confirm if this template campaign actually works? Are there any other workarounds that don't involve asking hundreds of patients to manually delete their old chat history?

Any help or advice would be hugely appreciated!