Bringing this issue back... first I thought it was a regular expression issue; now after going deeply, I see it appears because of this line under bb_func_txt.php (function textFilter):
$text=str_replace(array('."', ',"', '-"', ':"', ';"', ')"'), '"', $text);
So you may also enter as an alt tag:
hello; or
hello- or
hello. - the last character will be cut off.
Why does it happen? I have no idea. This string was introduced somewhere in the version 2 demos and I forgot already what it means. But probably it works something out.
So you can remove the parenthesis from
str_replace function and it should work for you. Unfortunately I have no other ideas how to fix it - except you should not use parentheses in ALT tag since this is very, very rare...