After the menu icon and progress window indicate that insync has finished syncing, some folder icons will sometimes still show the “syncing” icon for at least several hours. I have noticed this happen on both Mac and Linux.
Are the affected files always the same or does it vary each time you observe this? Also-- whenever this happens, do you quit > restart Insync?
Quitting / restarting Insync didn’t help. However, I noticed that after this happened the Insync Finder extension had gotten disabled somehow (it seems to disable itself frequently since I upgraded to Mojave), and re-enabling it then restarting Finder fixed the problem.