Light height

Is your fans to close,? Could be wind burn too, point fan at the walls 👍🏻 good luck 🎄

You can get a heat sheild to fix to the hps fixture, and if you add some leaf coat foliar spray will add some protection, but invest in a heat sheld for hps they work 👍🏻

The back of the hand trick is a good indicator for HPS light height, I run my 600w HPS at 35 cm above the canopy as that’s where the warmth feels comfortable on my hand . Your conditions in the tent sound a bit cool for a HPS grow unless you have 10c supply air it’d be hard maintaining under 25c with the lights on . What your describing deffo sounds like heat stress but the conditions sound great and your light is too high if anything. Try running the circulation fan facing up from the floor toward the light to blow the heat away from the canopy , that drops my canopy temps down a few degrees.

They can handle the light but not the heat and 55cm is close enough no need to burn them ....also HPS does work great but consumes alot more power than LED ...and with HID LEDs though the the spectrum is not perfect can produce similar results ...

The temperature in thebgrow box is 24°c max. it never goes higher than that and the fan works constantly , also it never goes higher than 25°c at the top of the canopy.

Grannie is right they are getting stressed and burnt. Need some good air movement in there. I have yet to use HID lighting, heard great things about it but I don't like how hot they get.

Because, my friend, they're getting too hot! I would get them up at least to 24" as you won't have more than a few centimeters left for them to grow before they start burning again... Don't try to lower them - as you've seen, you're just cooking the leaves! If you need to lower them for whatever reason, you can always do some LST - pull the stems down and away from the lights... anything to keep them at least 55 cm away from the light! Good luck....