I m having the same problem since months. Looked for something wrong on my phone and noticed that:
- my whatsapp use to act like a web whatsapp, meaning I dont see last seen for contacts that didn't registered me on their phone. It can last for hours or days or weeks. I found that last time this behaviour stopped correspond to hour of modification of three folders on my android/data folder, meaning: com.qapp.secprotect, com.qualcomm.qti.telephonyservice, com.qualcomm.shutdownlistner.
- when I open and close flight mode, the webwhatsapp feature disable for few minutes or hours. Im not sure, but it seems that messages that dont erase correspond to the messages that I send or receive during these gaps after trying flight mode.
-I use to send messages that are supposed to get to their destination (double gray check), but the contact never see them. when I open the chat after hours, the contact join the message
-I have lots of people with a mention that the chat is secured by encriptyon, as if I had a chat with these people, but I never had.
-when messages disappear, i noticed that it uses to have a confusion in dates in the remaining messages, like 17 of january, then 18 then 17 again
I mentionned it all to whatsapp support, but no answer
Do you notice the same things?