I am using Lightroom CC 2015.10 and Mac OS 10.12.4, but I think that this behavior has been present forever in Lightroom for Macintosh.
Lightroom "closes" the text box for all metadata when the focus is changed from Lightroom. This is different behavior than almost all other applications on Macintosh, which close the text box when an enter or carriage return (or mouse click) moves the focus out of the box. This makes it very difficult to copy and paste metadata from other programs into Lightroom.
My major problem is that I use a 3d party clipboard manager (Keyboard Maestro). I load up a number of text strings into different "clipboards" and I want to paste from them into Lightroom. But the moment I choose a clipboard to paste from, Lightroom loses focus, and closes the text box, so that the paste fails. I have to go back to LR, click on the text box, paste using the Lightroom internal command-V. This is different than every other program I own, and a big frustration.
But here is a frustration that everyone can experience: try to paste the lat/lon coordinates from Google Earth into the metadata of a photo in Lightroom. In Google Earth, you can only copy the Lat or Lon. When you try to paste the Latitude into Lightroom and then go back to Google Earth to get the Longitude, you get an input verification error in Lightroom as it tries to close the lat/lon text box. L.R. then clears the text box, so when you return with the Longitude, the latitude has already been erased.
The only way to paste Lat/Lon from Google Earth into Lightroom is to 1) Copy Latitude in G.E. 2) Paste Latitude into a text editor, then type a "," 3) Copy Longitude in G.E. 4) Paste Longitude into a text editor (since it doesn't close the document when you switch to G.E.) 5) Copy the entire string in the text editor 6) Paste into Lightroom
This is an unnecessarily convoluted process, made necessary only because Lightroom has different behavior than almost all other programs on Macintosh.