Security researchers found a bug in WhatsApp that could be used to crash the messaging app in a loop on the phone of every member of a group. The effect could be obtained by modifying a participant’s phone number and sending out a message. As a result, members would no longer be able to access the group or conversation history. The issue, fixed with the release of WhatsApp 2.19.246, was in the XMPP (Extensible Messaging and Presence Protocol) responsible for instant messaging.
Source: https://www.bleepingcomputer.com/news/security/bug-sent-whatsapp-into-crash-loop-caused-chat-history-loss/