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Should I be worried or is this a known bug?
This has happened all all three sites in this account which vary in shades of light grey :)
Until 2 months ago, the site had shown anywhere from 2 - 20 quality external links for every page. No it just says "no links found"
And not just for external links, but for internal links too, even though all my pages are linked within the articles in various ways. Same thing - just "no links found"
The external links are still out there - I'd written them down when they were showing. They're just not there in the Webmaster Tools.
And yes - no changes in ranking or traffic that I notice.