When you export a TIFF or JPEG with Color Space: AdobeRGB (1998), LR 5.7.1 includes an incorrect value for EXIF:ColorSpace. It should be set to Uncalibrated. But on Windows, it is set to sRGB, and on Mac it is missing entirely. (Note that when Photoshop CC 2014 saves a file in AdobeRGB (1998), it correctly sets EXIF:ColorSpace to Uncalibrated.)
This bug probably has very small impact in most cases, since it's likely that most color-managed applications ignore EXIF:ColorSpace when an ICC_Profile is present. But this did cause confusion for one person examining the color space with Windows File Explorer, which shows EXIF:ColorSpace but not ICC_Profile in its Properties window:
https://forums.adobe.com/thread/1806639