I finally got around to doing some comparison tests with and without a Schneider 4x4" IRND 1.5 that I bought on eBay from a reputable seller several months ago. I was surprised to find a very pronounced magenta tint. In direct sunlight at 6000K white balance on my GH4, it had a cold, reddish tint. In auto-WB, I can see the white balance changing substantially when I add the ND, and it still has a reddish tint. I've uploaded the video from my testing here:

Again, I bought the filter from a reputable seller on eBay with good ratings, it came in a pouch with the Schneider logo, and the filter itself has "Schneider" engraved on it. I bought it together with a Schneider Platinum circular polarizer that works great, which makes it even stranger. Could this be a counterfeit filter?