I asked this question under the display forum and basically the response is that Windows isn't doing things right.
I am getting lots of slave not found errors from I2C devices but only when the display is connected to I2C1 (pins 3 & 5). I have this with multiple Pis, displays, and sensors, so I can rule out any one specific hardware unit being at fault, and I have this with both RPI2 and RPI3, and the I2C devices all work without the display, even if I have multiple non-display I2C devices connected. Connect the display, even if I don't have code using it, and things go downhill quickly.
It appears the right way is through IOCTL (https://social.msdn.microsoft.com/Forum ... WindowsIoT), though I only see C code so far and haven't been able to figure out how to get a C# version built.
Can anyone shed more light on what is happening or the right way to get the display brightness set without messing up the I2C1 channel?