Hey,
I have similarly experienced sync failure of 60fps video show on LED wall and also on projectors .
You cannot recognize the sync failure In most case of slow motion of video show though,
that does not mean the video show is going on perfectly synchronized.
Once the show goes fast with such as X-Sports scene or funky video effects, it sometimes come recognizable on the lines of junction (of projection or LED cluster)
I would say the disparity only matters of one or two frames at top among the 60frames however, once it comes recognizable, it would never be solved nor ignored by itself.
Performance of all the computers run WATCHOUT above was quite good enough.
After all attempts and struggle, I am now assuming that the sync over the WATCHOUT network cluster works better than the sync within a graphic card.
I would try with two separate WATCHOUT computers and pull the video channel one from each.