For whatever reason some ascii characters make all your text disappear after them.
I think you meant "some
non-ASCII characters", surely?
The ASCII set contains 33 non-printables and 95 printables (62 alphanumerics and minimal punctuation). All those 95 should have no problem on
SMF.
As far as I've found, it's the unicode / UTF characters
outside the ASCII range that cause SMF to truncate the post: likely because unicode characters contain null bytes, and in string libraries that aren't unicode aware, such a null byte indicates the end of a null-terminated-string. So vowels with accents over them, fancy apostrophes, slopey speech marks, etc. all the fancy non-ASCII symbols are telling the string library "I'm the last character in this string".
It would be very nice if the Preview render alerted the editor to the fact!