This is not a fun flex to replace considering it is adhered to the front display, and removing a working front display for a soft key menu flex is difficult for most.
This is an odd case since you were not working near that area really and were not doing a display assembly. If you have another menu flex handy, plug it in to the device (without installing the piece itself) to see if it functions properly. There is always the possibility that your menu flex went out, but the probability of that happening is pretty low. That's why I would suggest the menu flex trial without installation to eliminate the phone hardware as being responsible.
Good luck!