Depending of the level of information test result summary changes:

On dashboard plan reports: 519 passed
On plan summary (cliked the plan name): Recent history contains for each row: Testless build
On build (chain) summary (clicked latest build result #): Tests tab contains: 519 tests in total 18 minutes taken in total

I tried to look at dotnet plugin source but couldn't figure out why there's a missing link in chain of results.