So this is a new issue to me since coreelec 19 arrived.
I guess I tried all version now (including rc).
Till 19.1rc4 I actually mostly got a pink(ish) or sometimes green(ish) picture. After the upgrade today (19.1) the screen stays black (no colours any more ). The black actually comes from my core elec box (s905x3) otherwise my TV would show a no signal sign an turn off automatically.
I don’t remember any problems with coreelec versions prior 19 and have my tv/kodi setup since ever on a socket (and the x96 box is quicker turning on then the TV). It always worked in the past .