I just checked and it behaves just as expected. The reverb of a hard panned signal is still going to be spread over the entire stereo field - that's just how diffusion works - there might be reverbs which under / overemphasize this effect, but this is not a bug.
And the signal is not MONO - slap a gonio meter on channel 3+4 to see the stereo field.
@Natan I'm using last develop version. A snippet won't help much because it's not a script-related function, but here is the option you have to enable before compiling HISE :
ReadOnlyPresets.jpg.
Once done, just save your factory presets within HISE and they will be automatically set as 'Read-Only' when you compile your plugin.