I don't know any details on how code coverage works in the org or in Welkin, so i don't' know if this is a bug.
I ran tests in my test class and was able to get my other class up to 100% code coverage. I then went into the Developer Console and cleared test data. I went and viewed the class in the org and verified that it said "Code Coverage 0% (0/29)". I went back in Welkin and clicked "Refresh" in the Code Coverage panel. Once it was done, it still showed 100% coverage for the class.