TandemGeek mentions tight clearances. How much space do you have between fenders and tires in your winter fender setup?
Just a couple millimeters in some spots, perhaps 5mm at the most.
Not ideal for riding over freshly paved roads, but given we ride slicks and don't ride off paved surfaces with these particular tandems it's enough. The front mudguards have break-away stays as well, so if some wayward stone did get picked up by the front tire and make its way between the tire and mudguard, the mudguard will pop loose before the wheel's rotation is affected.
Again, this underscores why mudguards needs to be installed in such a way that they don't have any slop or movement.