upvoted for the old but quality reference :)

So it's all sitting on a layer of small rock so water would drain well. And I'll be making a slope in the concrete so the water should get right at the end of the pad where I'll be making a franch drain / rock walkway to make sure nothing pools too much. Additionally my roof is overhanging quite a bit so the ~40cm are most of the time dry when it rains anyway unless rains gets sideways twice every 5 years or so.

Concrete can dry out too fast before it sets or cures if you don’t. It prevents the subgrade from leeching the moisture out.

Could be other reasons at play here though.

yup. Need to get the outdoors more practical as our weather is now all hot & dry...