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The range of generate for symbols is only from abcdefghijklmnop as documented here: https://code.kx.com/q/ref/deal/#roll-and-deal
As to why is that so, I have no idea as well.
But you can use something like this to solve your problem:
q){[num;len] neg[num]?`${.Q.a cross x}/[len-1;enlist each .Q.a]}[17;1] `e`n`d`g`r`y`b`p`j`l`s`i`f`k`w`u`c q)asc{[num;len] neg[num]?`${.Q.a cross x}/[len-1;enlist each .Q.a]}[256;2] `s#`af`an`aq`as`ax`ay`ba`bd`be`bj`bk`bl`bn`bo`bu`bw`ca`cb`cc`cd`cf`cn`co`cq`c..