autocmd only works once after upgrade to 0.10.0 about neovim

[clason]

Any return value will delete the callback, see :h news.

Your example will not work correctly anyway, since the callback is called with arguments, which other functions will not expect.

Callbacks should always be wrapped in dedicated functions; do not simply pass handles to API functions.

[telbizov]

Roger that.
I had this code piece before and it used to work but I am far from an expert in neovim and Lua. Now that I took a closer look the following is what it should be:

vim.api.nvim_create_autocmd("CursorHold", {
    callback = function(ev)
        vim.diagnostic.open_float()
    end,
})

This works now!

Thanks for the pointers