https://www.bottompaintstore.com/blog/tag/paint-chalking/

I think what happened was you got frosting problem.

FROSTING:
A white, salt-like substance on the paint surface. Frosting can occur on any paint color, but it is less noticeable on white paint or lighter tints. On masonry, it can be mistaken for efflorescence (see Efflorescence and Mottling).

POSSIBLE CAUSES: Forms mostly in protected areas (such as under eaves and on porch ceilings) that do not receive the cleansing action of rain, dew and other moisture. Use of dark-colored paints that have been formulated with calcium carbonate extender.

Application of a dark-colored paint over a paint or primer containing calcium carbonate extender.

SOLUTION: Frosting can be a stubborn problem. It often cannot be washed off readily. Moreover, the condition can recur even as a bleed-through when a new top coat is applied. In extreme cases, it can interfere with adhesion. The best remedy is to remove the frosting by wire brushing masonry or sanding wood surfaces; rinse, then apply an alkyd-based primer before adding a coat of high quality exterior paint.

Removal by sanding down to bare cement plaster, wipe down whole wall using bucket and some soap solution with old t shirt/rags until not powdery, then use primer (nippon 5100/5400), then finish topcoat (colour paint x2 coat) will solve the issue. Looks like because old paint still there the problem never go away.

Basic sander without dust bag, uses sheet sandpaper (the rougher the better)
https://www.lazada.com.my/products/corated-...2-s2719085.html

Atas sander with dust bag:
https://www.lazada.com.my/products/bosch-ge...s170092766.html
https://www.lazada.com.my/products/dewalt-d...s100004558.html

both will create iraq desert storm level of dust, move out everything from the room before start sanding.
Hand sanding will be extreme level workout.
Would recommend at least the basic sander plus maybe 20-30 sheets of sandpaper. Even bosch brand sandpaper only RM1/ea at CSM 225 PJ