Is it possible for an HDMI cable to be "worn in"?
Something else must have changed that causes the signal to reach TV better. Maybe TV firmware got updated and new input equalization settings updated with it. Maybe computer video card driver got updated and it uses more drive strength or pre-emphasis for the signal. Maybe some other devices are off so that there is less interference. And thirty feet is on the edge of being able to carry 18 GBPS 4K HDR signals reliably anyway, way more than many devices suggest in their manuals.