@EXECSTR line length limit? ... misbehavior?

Outside TCC, I've been playing with MAPM, the arbitrary precision math library that TCC uses. The command ipow.exe 500 5000 correctly sends the number 500^5000 (13495 characters) to stdout. I thought that would be OK with @EXECSTR but echo %@execstr[ipow.exe 500 5000] produces 13506 characters, and among those characters are 00000000㘴ㄵ〵㠷㘱㌸㔶㘸抵쁎鹷00000000.

Any idea what's going on?