The oh-my-fish installation sets the default theme after installation which uses the right prompt to display the clock (probably if there is no right prompt defined). This leads to an unusable shell on Windows with ConEmu and on Linux with GNOME Terminal (see fish-shell/fish-shell#4306).

The underlying issue here is, I think, that the oh-my-fish installer should not enable any kind of theme until specifically asked (when a user installs a theme and oh-my-fish enables it, it is fine).