Yes, would i be correct in saying the crack is in line with the tip of the guide foot?
If so I would hazard to guess that you have a few millimetres of thread up the blank from it?
If that is correct, then in the future, only have three turns of thread before it starts to climb up the foot. When adding epoxy, only overlap onto the blank a fraction, no more than 1mm.
That should sort your problem as the flexcote should be able to bend more evenly.
As you already have noted, the flexcote does remain flexible, but the guide foot acts as a stress point. It's a well known problem on big casting sea and coarse rods butt rings.
If, however, it is not cracking at this point, but further up the foot, then it may be down to the gap between thread and blank being a void rather than filled with epoxy.
Hope that puts you on the right path...