Is my laptop charger is working as harmonic filter and absorbing harmonics?
All switched-mode power supplies have filter capacitors on the input.  These are mainly to stop electromagnetic interference escaping from the power supply, but they will also reduce any voltage spikes and harmonics coming in.

It may be that the filters on your laptop power supply are just enough to round off the edges of the square wave from the UPS.  They may be working hard to do this, and they may fail eventually.