You are right, it's too short for 6 weeks of age. My best guess: light was too close therefore plant couldn't grow taller. All other parameters seems to be within norm and your plant looks healthy.

I am definitely no expert - quite the opposite, I’m a complete beginner! But I’m 7 weeks into my first grow and in week 4 and 5 my plants were still fairly small and my expectations were pretty low. Then all of a sudden within a week or so they easily doubled in size. I was having to adjust the lights daily by about 10cm ish. I can’t offer advice but I had the same worries as you and now the plants are so much bigger than I ever expected. Good luck with them :)

The stretching of the plant is caused by light. Your plant has enough light that it doesn't need to stretch to get it. If you want your plant to stretch a bit, just lower the light intensity or raise your panel. Although I wouldn't recommend doing this. Since your plant will stretch enough during the flowering stage.