strange field

Dear Steven,
I have one question.I noticed when i run simulation for square rods(from examples folder) that if the rods are put in the center i get strange field distribution , but if I shift the rods just a little bit, the field turns normal. This is input for centered rods:
(set! num-bands 2)

; Definizione vettori reticolo reciproco
(define-param Gamma (vector3 0 0 0)) ; Gamma
(define-param X (vector3 0.5 0 0))   ; X
(define-param M (vector3 0.5 0.5 0)) ; M

; Definizione zona di Brillouin
(set! k-points (list Gamma X M Gamma))
(set! k-points (interpolate 8 k-points))

(set! geometry
	(list
		(make cylinder
			(center 0 0 0)
			(radius raggio)
			(height infinity)
			(material
				(make dielectric (epsilon 8.9))
			)
		)
	)
)
(set! geometry-lattice (make lattice (size 1 1 no-size)))
(set! resolution risoluzione)

; (run-tm)
; (run-te)

(run-tm (output-at-kpoint Gamma fix-dfield-phase output-dfield-z))
(run-tm (output-at-kpoint X output-dfield-z))
(run-tm (output-at-kpoint M output-dfield-z))
(run-te (output-at-kpoint X output-hfield-z))

for shifted there is only difference in:
(set! geometry
	(list
		(make cylinder
			(center 0.099 0.099 0)
			(radius raggio)
			(height infinity)
			(material
				(make dielectric (epsilon 8.9))
			)
		)
	)
)

I can't understand why this happends. Could you please explain me why?
thank you!
jure grgic