The LR white balance eyedropper tool uses a fixed (non-adjustable) sample size of 5x5 (25 pixels), regardless of the loupe size setting. The white balance reading is the "average value" of these 25 pixels summed together. My guess is if any one (1) of those 25 pixels is at 100% RGB value, LR rejects using the sample. This is probably why there is no change to the white balance with the high 90s samples, but I can't explain the History entry as 'Camera RAW'. What do you normally see with TIFFs in the History panel when making white balance corrections with the eyedropper tool?
I'd go a bit further with this line of thought: A WB correction is done by boosting the red and blue channels relative to the green. If the sampled levels are so high that the red and/or blue values would be pushed to clipping, LR won't do the operation and the caption "Camera Raw Settings" indicates that the WB performed at the time of conversion from RAW has remained unchanged.