Rain water can be quite acidic.
Palm peat = coconut husk fibre? You may have troubles related to high coco content if that is the case (it prevents Calcium uptake by scavenging and locking it) and will want to make certain you water with calmag to prevent this.
Rain water= NO mineral content at all and needs calmag added, period. It is also frequently acidic from atmospheric N and S gases that go into solution as their respective strong acids. Calmag WILL help this; add it to 300 ppm and then test pH after addition. ADjust to 6.5 for your soilless mix.

The damage looks like a metals lockout or deficiency. I'd raise the lamp just a bit to let the plants get established, and do a LIGHT foliar feed with the 'micro' part of the GH 3-part, or a comparable micronute supplement. Algoflash regenerateur is one that should be easy to find in Europe (you give temps in 'C... lol... I assume you're on the other side of the pond).

Also next time you mix up media, remember:
-If you use peat moss, you must add some lime to balance out the acidity.
-Soil lighteners such as Perlite are very important

Another possibility is that the new growth of your tender young clones is simply getting scorched by going IMMEDIATELY under the big lamp. I like to let my clones root up nice, and then actually leave them under flourescent shop lights during the hardening-off period, about a week, during which time I remove the dome and mist them with SMC lightly to get used to the drier surrounding air.