API markers in builds are flaky

[jukzi]

Since API warnings in builds are enabled (most) PRs randomly marked as unstable.
While the ECJ markers seem to work fine and stable some API marker are recognized as "new" even though there are very old.

Example jdt.ui:
Both build 44 and 45 show the same marker "ViewerPane illegally extends ViewForm" but 42, 43 did not

https://ci.eclipse.org/jdt/job/eclipse.jdt.ui-github/job/master/44/eclipse/source.c6d032a9-4965-47dc-a9b6-cd5907c54ac1/#28
rendering 44 and eclipse-jdt/eclipse.jdt.ui#1074 (based on 42) as unstable
https://ci.eclipse.org/jdt/job/eclipse.jdt.ui-github/job/PR-1074/21/

Example platform.ui:
https://ci.eclipse.org/platform/job/eclipse.platform.ui/job/PR-1742/4/ unstable because of "new" but ancient API errors

@laeubi any idea why?

If we can't fix that i would rather not like to see API errors tight together with the stable ECJ markers.