[arboles]

The following:

  \~300 lines

Still exports as <p>\~300 lines</p> (in case of an HTML export). In fact you could "escape it" with any character, the backslash isn't doing anything special, the formatting just doesn't trigger if there's no white space on one side. When I use Org I also do not have auto-formatting, as you do, but the highlighting would still be wrong. And it'd be masochistic to put up with that unless you've invested a lot into Org for other reasons.

[skeledrew]

Sounds to me like it's a shortcoming of the exporter then, to not remove the escape characters as appropriate. Though one would think others would've run into this issue and fixed it. Or perhaps you're missing some other prevalent alternative?

The highlighting corrects for me once I "escape" the "~300".

[arboles]

You didn't understand me. The highlighting also disappears if you "escape" with %

  %~300 

Or the character A:

  A~300