Curious to see if there is a way to look at a lead record and tell how many lists or smartlists that lead is a member of. The lead sync is not happening because of one smartlist, but that smartlist is made up of many nested levels of smartlists and static lists. If I can see all the lists the lead belongs to, then I can narrow my search for the faulty logic.
there is not. You can do this for Segmentations, however, but only on the Lead Detail.
Instead, why not unpack your nested lists. It's not a good practice to nest too many smart lists for exactly this reason, as well as speed.
I would seriously consider rebuilding your Ok to Sync smart list instead. I've seen this issue many times.