I'm not surprised about the four I got in a trade, but I had all my chained shinies checked once you told me about the Caterpie, and two others came up that way too. From what I can tell researching online, there seems to be something about some (very few) chained shiny PID's that causes them to fail the sync test, but I don't know enough about the game's coding to figure it out.
I can swear up and down that Caterpie's legit and a caught it myself, but I don't expect anyone to believe me over legal.exe. ^_^ I would consider it a hack in your place, too. I'm just bummed that now I have to go chain myself a new one... it wasn't an easy chain...
At any rate, I'd like to replace the 5 that failed a hack test with ones that I caught myself and know are legitimate. And thank you for letting me know. I definitely do not want to trade hacks, or anything that looks like them...
Sorry for the trouble.