frac is the number of decimal digits after the point
For each multiplication in the expression, decimal_mul() does this:
  to->frac= from1->frac + from2->frac;              /* store size in digits */
which will eventually overflow.
The code for handling the overflow, will truncate the two digits in "1.75" to "1"

Solution:
Truncate to 31 significant fractional digits, when doing decimal multiplication.