https://thenextweb.com/security/2019/09/18/whatsapp-delete-everyone-android-iphone/

It turns out WhatsApp‘s ‘Delete for Everyone’ feature — designed to let you wipe texts and files you sent by accident — could still leave traces of your messages, due to a discrepancy in the way the function works on Android handsets and iPhones.

Researcher Shitesh Sachan noticed that even after purging messages, media files sent to iPhone users in group chats (who have enabled the ‘Save to Camera Roll’ setting) will remain on the device.