i’m having the same issue on Ubuntu 24.04. lxc-attach (version 5.0.3-2ubuntu7.1).
When there is a lot of output, parts of it are dropped. It looks like a flow control issue. TUI applications like vim tend to be mostly unusable.
FWIW, opened an issue on the github: lxc-attach drops output with lots of console output · Issue #4546 · lxc/lxc · GitHub