Changing major mode in composition buffer erases header line and binding

From @wolfjb:matrix.org:

> using M-RET to compose in a buffer, then using M-x org-mode to switch the major mode, the C-x C-s keybinding disappears and the message doesn't post. But copying it, killing the window, and then M-RET again does show the keybindings and pasting the message works.