Interesting, but given that the code in question computes (expt 2 488) as the square of (expt 2 244), is there actually any way for that bug to cause the observed symptoms?
(2^244 is being shown as zero. 2^488 is being shown as non-zero; presumably the right value though I haven't actually checked. If 2^244 is actually being computed as zero, then unless what's happening somehow depends on the depth of the stack -- surely not possible if this bug is the cause -- 2^488 should be computed as zero squared, which is zero.)
But the sequence of multiplications chosen to reach the exponent isn't different. What the user's expt function does, when asked to compute (expt 2 488), is to compute (expt 2 244) and then square it. When it computes (expt 2 244) it does the exact same sequence of multiplications as if you just asked for that directly from the REPL.