Question about the use of mfslashexpansion

Hello,
I was trying to understand the use of mfslashexpansion via the following example but it is giving me incorrect answers:
I am trying to compute the action of eta(z)^8 * eta(2z)^8 acted by U_2 which equals -8 * q + 64 * q^2 - 96 * q^3 + .... On the other hand,
a = mffrometaquo([1, 8; 2, 8]);
b = mfinit(a);
c = 8 * (mfslashexpansion(b, a, [1, 0; 0, 2], 50, 0) +  mfslashexpansion(b, a, [1, 1; 0, 2], 50, 0))

[0, 0, -8, 0, 64, 0, -96, 0, -512, 0, 1680, 0, 768, 0, -8128, 0, 4096, 0, 16344, 0, -13440, 0, -8736, 0, -6144, 0, -11056, 0, 65024, 0, 20160, 0, -32768, 0, -117648, 0, -130752, 0, 319520, 0, 107520, 0, -97536, 0, 69888, 0, -549696, 0, 49152, 0, 272200]

I looked at the documentation but I am not sure what am I missing. I am encountering similar issues on slash action by H_5 = [0,-1; 5,0]. Could someone please clarify this?
Regards,
Swati
"An equation for me has no meaning unless it expresses a thought of God"— Srinivasa Ramanujan.