Performace analyzer gives bad readings for jets on Sergeaa at high mach

I used an engine with 200% size(not tinker panel) and max bypass ratio and compression(all stock). As I was analyzing the performance of it on Sergeaa I noticed that at 1.3 mach at 0.0 km Altitude the ISP, delta-v and burn time in the stage analysis said 0 and the ISP in the analysis for the engine itself said N/A, while the thrust went instantly to just above 0 N. Just one step of the mach slider below 1.3 mach, at 1.29 mach the ISP read over 1000000 and everything else around it seemed to me like it should make sense relative to the ISP reading.