I don't know why that is happening but just save the file and then re-open it.  The letters magically re-populate.  It is happening in the individual program as well.

YMMV, but I have noticed the client letters formset disappears AFTER you print the return, either on paper or to a file.  I have not, so far, had any issues printing any letter BEFORE I print the actual tax return.

Regardless, as mentioned above, the formset reappears if you close and then re-open the file.