After the October CC update to After Effects (12.1.0.168) I am experiencing multiprocessing RAM-preview render issues in compositions regardless of presence of any effects or of the format of the media. This manifests itself as intermittent flashing "offline media" frames during the Ram Preview playback, and sometimes during the ram preview render. When Ram preview playback is paused those of the frames that exhibit such an issue are shown as not rendered in the Ram preview indicator on the composition timeline. Same timelines, when previewed without the multiprocessing enabled do not show such issues.
Compositions set-up in the similar fashion in After Effects CS6 do not show such issues with or without multiprocessing enabled.