AFAIK the basic edit controls in Windows are limited to 64kb of data. I think only the richedit controls are able to store "unlimited" data. mIRC uses the standard edit controls. This may explain your predicament.
Can you tell us how big the file is, in bytes, as well as how big the file *would* be if it was only those first ~655 lines? If it approaches 64kb that would be the likely cause.
Also if you have any $chr(26) (the actual character, not the identifier) characters in your script they will cause /loadbuf to stop reading