H264 3D videos seem to have unsynchronized frames

[Firepal]

Playback is not synchronous with both eyes, the right eye will get a later/sooner(?) frame than the left eye.
On scene changes this is exhibited as a small stereo "flicker" as one eye sees something completely different than the other.
(I currently only tried 3D videos with H264.)

This is my conversion command, a version of the README conversion script that only calls ffmpeg once.

ffmpeg -i <INPUT> -lavfi "[0:v]scale=800x240:flags=neighbor[scaled],[scaled]split[x][y];[x]stereo3d=sbsl:ml[l];[y]stereo3d=sbsl:mr[r]" -map [l] -map [r] -map 0:a -ac 2 <OUTPUT>