This problem appeared after the way masks are handled (https://bugs.webkit.org/show_bug.cgi?id=139294). It seems like sometimes the tester takes the snapshot at a time when not both before/after masks are loaded and, as a result, the blend between the url A and url B masks is not cross-fade(url(A), url(B), progress) but simply url(B).
* after the way masks are handled WAS CHANGED.
The fix for bug 139294 was rolled out, so this problem doesn't occur any more. Keeping this open in case this needs to be addressed separately before the fix can be re-landed.
Bug has been re-marked as flaky (patch for bug 139294 was relanded as the rollout was due to a pre-existing problem - see bug 139644)