It may be a reaction to the stain I havn't heard of this wood but that would be unlikely, it may be the nails used to secure the beads , copper , steel or zinc have differing effects on deiffent woods , I would guess though if the staining is mainly on the horizontal beads it is from water ingress . Quite often these beads are put on with putty on a roll , this tends to stick to the glass but not the bead in addition it may be badly trimmed . check carefully to see if there is any crack between the putty and the bead . If you have no joy here try phoning Sikkens technical help and ask them .
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