Thats weird.. maybe its a memory corruption problem or something with the Thingspeak arduino client. Or maybe the total length of the data being sent exceeds some preset limit and gets chopped off.
Is it that it only sends field1 to field5 or will it work with, say, field3 to field8?
Unfortunately Thingspeak doesnt have a debug console so you can view. You might try saving the return code from writeFields() and printing it out to see if you can spot a pattern
Otherwise you might be forced to hack into the source code. According to https://github.com/mathworks/thingspeak-arduino/blob/master/src/ThingSpeak.h if you locate the file ThingSpeak.h on your computer and add
#define PRINT_DEBUG_MESSAGES
Near the other define statements it might produce some helpful comments.