I'm a heavy WebK user and I've noticed several times (especially when reading a backlog of messages) that not all messages are displayed.
Usually I notice when I see someone replying to a message which has not been shown to me. The message-teaser in the reply is shown, but not the original message itself. When I click on it, then WebK notices and seems to reload the chunk around it (usually more than that one message, I'd guesstimate between 3-12 messages suddenly appear)
Just today I had a chat where the last 3 messages did not show on WebK, even when doing a reload / forced-reload of the browser. I could see them on my other devices. Only after I've written a new line myself did WebK update and show them.
This is very concerning, since missing messages might be important, or at least rude if they "are ignored". And if they don't happen to be replied to by someone, I wouldn't notice that they're missing at all.
I don't post screenshots, as this woul contain private chat data, and is not reproducible on my end. But happened often enough.
I don't know how to reproduce :(
Telegram Web 1.5.0 K, Chrome 103