No problem Rocky, you've brought up a flaw that will happen from time to time. All replies to that post on Modern will basically generate their own thread on classic. Then replies on classic to that thread, probably will become new threads on Modern. It's a hopelessly out of sync condition that I never considered.
It's easy to do though, because you can search up an old thread on Modern or get there from a search engine, and there is nothing to stop you from replying to it.
I'll think about how I might keep it from happening, but it's likely going to take some programming to stop it.