It seemed to be a boundary error. (point) was at the end of the
timestamp which doesn't hold the org-appt-warntime property.

foo.org illustrate what I have seen. Evaling the get-property sexp
returns nil.

The patch adds a function that goes at the beginning of the header to
get the property. As a bonus it turns the string into a number.

Please let me know if the patch is accepted or needs improvement.