Hi Eric,
In what way is the LTS Schmitt 'non-standard'? It is a generic part in the standard Digital library provided by LT with the LTS download. The Help file clearly indicates that its values Vhigh, Vlow, Vt, Vh etc are customisable (I assume to allow for different logic families, e.g. to cover the 3-15V supply range of the CD4000 family); so presumably the simulation correctly takes custom values into account. Why should that cause problems? It is perfectly valid to use 4000 logic at 4V.