In AC, diodes work half the time, every 1/60 second. The “good” LEDs will have circuitry to fully rectify the AC into DC, drop the voltage properly, and smooth the peaks and valleys, so they will be continuously lit. So the cheap LED Christmas lights might have a slight flicker, and the good ones are steady. (Or get fancy with chasing colors, etc.)
All of that happens inside each of the “bulb” enclosures, or sometimes in a box at one end, so it technically doesn’t matter which end they are getting electricity from, since the socket at the far end is still just connected in parallel to the plug at the near end. (Otherwise you wouldn’t be able to link them together.)
It’s just a really bad dangerous idea to reverse them.