I guess, it's about $ expansion issue and not a bug. Try using any date function while defining the varibale like:
LET vQVD_TS = MakeDate(2018,2,15) // instead if '15/02/2018';
It should work fine then.
LET vQVD_TS = '15/02/2018'; // this doesn't work potentially because, with $-expansion, '/' could be interpreted as division operator. And that gives you a different unwanted number.