The only things I can think of are:

a) There is something else, eg. the nut, causing the string to snap but I'd have thought you know stringed instruments well enough to have checked for any mechanical problems.

b) The scale length is too long. My 17 fret tenor is 54cm/21". A 19 fret would be about 58cm/23". Both of these shouhld go to CGDA.