ᐅ Microcracks in Old Drywall Panels

I am renovating a house from the 1970s.
The drywall sheets in the living room ceiling are probably the same age.
I first applied a Q1 finish, then Q2, and have now fully skim-coated with Q3.

I am now noticing micro-cracks appearing in several places.

I used Knauf Uniflott Finish and did not prime beforehand, which had worked well so far.

What did I do wrong, and more importantly, how do I repair this?

Thank you

Prime the surface and apply a new layer. Sanding might also help.

However, it can be assumed that something else is damaged at this spot on the wall if it looks like this…