Hey buddy. This one has me too. It's very very strange. I can't see any reason why your otherwise trouble free very healthy looking plant would stop growing. When I have problem here's my go to site. It could be root probs, interruption in the dark period. Old seeds, many factors. Go through this list and hopefully you should find out why your healthy plant has stalled. Very strange indeed. Please post a comment or DM me if any of these issues could be playing a factor please, I always love learning about autos, as they are strange compared to photos arent as forgiving for mistskes the same mistake photos would breeze through, you kinda have to hit key things, also before you got your seeds to u keep them in 8-11c, i know they dont come through the post at that temp. But long exposer to higher temps to your seeds might be the only reason u can find,hope this helps; https://www.royalqueenseeds.com/blog-slow-cannabis-plant-growth-and-what-you-can-do-about-it-n961
Happy growing, good luck!