I would expect: math.eval("[1,bignumber(2)][2]") === math.bignumber(2). Unfortunately, access.js calls .valueOf on the subset result for matrix requests. It is the only case of a valueOf call. Bignumber returns the string result on a valueOf call. It's not clear why matrices uniquely require a base value on result. It would seem to me we could just remove that call.

Reference link: access.js