New growth crumbles

Many of my Pachypodiums is waking up from the winter. unfortunately the first leaves on some of them dryes out quickly and crumbles. The plants are in a window facing south, since it not warm enough to put them outside yet.

What do you think causes this?

Im thinking maybe it gets too hot in the window (It's a big window so it gets hot when the sun is on)

Maybe combined with too low humidity/not enough air circulation.

Or maybe not enough nutrients in the soil (Pumice/Perlite and some Organics)

Im thinking of setting up some sheer curtains to filter the sharp sun a little, and maybe get the temperature a little down. Or maybe move the plants a little away from the window.

any suggestions and good advice is welcome. Or if you have experienced something similar, what did you do too prevent it?