1. I look at it as poor workmanship during the wiring and plastering.

2. There are 2 possibilities ..... there will still be water dripping after repair and require few days to dry b4 painting or the leak wasnt repair. I suggest you ask the seller to redo and revisit the place in 2 weeks to see whether the same problem recur.

Fixing a leak is not easy. If the upper neighbour is not co operative, no leak can be repaired. Not every owner is nice enough to spend money and time to 'destroy' their flooring for another person. Some aggrieved neighbour have to go through 'legal means' to have it fix.

So you decide if the leak cont, it is worth to proceed ?

Thanks guys for the insight, I really appreciate it.

Since the unit above is occupied, there will probably be continuous usage of water, if the problem persists, the leakage stain would probably resurface.
I guess I'll hold off the purchase for the time being and tell the seller I would view the unit again in 2-3 weeks, just to make sure the problem is being taken care of. Normally how long should we monitor to determine whether the issue persists? 1 month from the repair period or much longer?