aggregator API is inherently racy
The following discussion from gst-plugins-good!711 (merged) should be addressed:
Hm, what if the pad gets flush stopped and receives a new buffer in the interval? In that case, we may end up not muxing the "best" pad. The alternative solution is to keep a reference to the "best" buffer alongside the best pad, and call drop_buffer() on it once actually processed.