To confirm this issue just log should_attach args in any of these cases:

• start Neovim with multiple files open
• use lazy loading (as shown in README), start Neovim, open multiple files without triggering lazy loading (do not enter Insert mode), then enter Insert mode

At a glance the issue happens because:

• util.should_attach checks a current buffer
• but it is called from background (it's callee runs under vim.schedule)
• it is called just once on start (in client.setup) while multiple buffers might exists at that moment because of lazy loading