Yes, this is a known bug in Lightroom CC2015.5: Lens metadata hidden for some cameras in Lightroom 2015.5 and 6.5
While it is reassuring to know that this is recognised as a bug, any idea when a fix will be available? The metadata is certainly there, because the correct lens appears in develop/lens correction and it is there in Bridge, but it not being used in the LR metadata--all combinations appear as 'Unknown Lens'.
Images that I imported on 15th March 2016 are fine; those imported after 21st March (nothing between these dates) are all missing this data.
In my case, Lightroom CC; Nikon D800 and D700; 16-35 f4; 24-120 f4; 24-70 f 2.8; 70-200 f2.8; 50 f1.8; SIgma 24 f 1.4; Sigma 35 f 1.4.
Adobe does not publish dates in advance. However, I'm sure the fix should be due in a few days.
An Addition: Photos taken with an Olympus OMD EM5 Mk II over the same period have the lens data where it should be--so it seems to be a Nikon issue?
Yes, this is a known issue mainly with Nikon cameras.