It seems to me that the light is too far away from the plant. Moving the light closer should make the plant a bit more compact and stretch the stem less, making the stem stronger.

I don't think that damage could be done in just 5 minutes of exposure to a hot light, but I haven't dealt with that so I am not sure. However, I see you have mentioned that you water with only water and no nutes, so it is not nute burn. I also see that you have mentioned that you water with 500mL of water. Is this tap water, bottled, home-RO filtered or anything else? Do you pH test water and runoff from the plant? As plants are growing and develop larger root systems and greater water needs, they need varying levels of water. Do you feel the soil after watering to ensure that it does not need more water? Does water come out the bottom of the planter after watering, signifying that adequate water has been given? Is there proper/adequate drainage either built-in or added into the planter?

I am not an expert and I don't have a TON of hands-on experience dealing with that problem. However, I have done a lot of reading regarding possible issues that I will face with my plants and I've seen a lot of similar issues that people have posted about in this forum. I would start trying to water the plant more. Water amounts depend more on the actual needs of the plant and not on a standardized measurement for the water that you are doing each watering (although it will also be based on the measurement of water that you provided for the plant during the last feeding). Even nearly identical plants that are the same strain, planted at the same time in the same type of soil may need varying water levels. Hopefully your issue is something relatively easy to correct like that!

Good luck, homey!