How to update WhatsApp after migration
Once you got back all your iPhone messages and media you MUST uninstall older WhatsApp and update to latest version from Google Play Store in order to get all features working
As a precaution before updating: make sure you disabled previous Google Drive backups since this could interfere with the restore and a WhatsApp local backup was performed (usually performed automatically after successful restore).
Now you can safely uninstall the old version and install the new one from Google Play Store (interactive tutorial below). You'll be asked to activate WhatsApp again, this is expected and not an issue (you'll not lose the imported messages).
Please note the screen below to make sure you restore from LOCAL backup.