WhatsApp chat restore to new phone is incomplete

Hello, thanks you for your help.

i followed the iPhone process to transfer from my iPhone 8 to new iPhone 12mini, but found some recent WhatsApp chats were missing.

So I went back to the iPhone 8, reactivated WhatsApp where I found all my chats and and backed up chats to iCloud manually; it is a large chat history about 10gb; then went back to new iPhone 12mini, reinstalled WhatsApp app from App Store and restored chats from iCloud.

Despite doing this I still find it to be an incomplete restore with missing chats.

When going back and forth between the phones I even moved the SIM cards each time in case. I see from the internet people have found a solution but for android phones, which involves deleting WhatsApp zip file but can’t do this on an iPhone.

Grateful for any suggestions, thanks.