I noticed this most recently on https://pinpoint-dot-chromeperf.appspot.com/job/12f57c74e40000 but I previously saw this on other pinpoint runs.

It looks like Pinpoint reused an existing build (based on the fact that the "Build" boxes very quickly turned green & had an isolate hash). But there is nonetheless a build ID, and it seems that it will actually rebuild it. It seems like it could save some resources by not rebuilding?