Current flow though Pin of ups
Neutral peaks may rise up to 10% of the line voltage peak under heavy load with minimum wire gauge, but more likely without earth ground, you felt the EMI cap leakage current designed to reduce inverter noise on the line via Y caps to earth ground.  Since your plug was disconnected without an earth ground all the SMPS leakage currents of your UPS and load (e.g. SMPS line filter EMI Y caps to frame which were not diverted to earth ground plug.  This is considered a safe current but depends if it was compliant.

Your UPS and PC PSU may have a discrete version of this line filter (aka Pi filter)


You can look up the ground leakage specs for your UPS and PC. They may be 0.25mA max each, I believe.  Check EN 60950-1 class ratings.

This is why the frame ground and Neutral will have leakage currents that will begin to flow if you become the earth ground path with stray leakage thru the moist air to earth ground.

Therefore if using it in this mode in future beware of this potential nuisance.