This is a syntax highlighting issue, most likely. The \ commands are not highlighted. Are you using a white or light colored background? Likely the contrast isn't high enough. I just updated the documentation with more information on controlling syntax highlighting.

You can either try disabling syntax highlighting completely ( \set SYNTAX_HL false ) or change the Chroma style used ( \set SYNTAX_HL_STYLE paraiso-light )

See this page: https://xyproto.github.io/splash/docs/all.html for an example of what the various Chroma styles should look like.