A bug? Or did I oversee something?

I created a question with:

a:rand_with_prohib(2,9,[0]);

b: rand_with_prohib(2,9,[0,a]);

c: rand_with_prohib(1,9,[0,a,b]);

d: rand_with_prohib(1,9,[0,a,b,c]);

When Question-level simplify = Yes => no problem

When Question-level simplify = No => I get this error:

The CAS returned the following error(s):b: rand_with_prohib(2,9,[0,a]) caused the following error: apply: subscript must be an integer; found: 1+1 c: rand_with_prohib(1,9,[0,a,b]) caused the following error: apply: subscript must be an integer; found: 1+1 d: rand_with_prohib(1,9,[0,a,b,c]) caused the following error: apply: subscript must be an integer; found: 1+1

I go back to "'Question-level simplify = Yes" and experiment further

Cheers

Hiram