Any ideas what cause this??

Nutrient burn.

Hi mate, Your room parameters are 25°C and 40% RH, but the good range for humidity according VPD (Vapour Pressure Deficit) is : 75-85%, at this stage of growing. I guess the plant are suffering of dry air, especially at this stage of veg. Hope this helps

This not a deficiency. It’s the EC of the soil becoming to high certainty because you’re not watering enough and getting no drain from the bottom of the pot. NaCl is starting to build up in the soil.

Did anyone water them when you were away? they were suffering from heat apparently but there is also a salt build up in your pots. I can only guess that someone watered with inadequate amount of water,without runoff. If not, this message will self destruct.