Which leaf pair is wrinkling, the old or the new one? Or is there only one pair?

I started to water most of my Lithops just a few weeks ago, and some of them still have not got any water as the old leaves have not dried up completely yet.

There is only one leaf pair. The old leaves were completely dried up when i got it.
Could this be a sign of sun burn or rot?

Where is the plant located? (Outdoors, windowsill, sun exposure)

What kind of soil is in the pot?

Sunburn causes tissue to blanch, not wrinkle. Rot will typically turn the plant into mush instantly. Wrinkling of the skin is a normal response to summer's heat and is not a cause for concern. Watering lightly won't harm the plant.

What will harm the plant is inadequate light, poor soil and a lack of awareness of the plant's growing cycle.

x

The plant is outdoors, it gets about 2 hours of direct sunlight a day. From 11:00 to 13:00.
It is still in the same soil it came with. It dries very fast so it must be some kind of cactus mix.

It has been very hot the last few days here so maybe that is the cause. I moved the plant to a more shady place now and will wait to see if this helps.