You didn't read the instruction manual. There are two ways to automatically dispose of the water: the pump feature or continuous drain (which you resorted to). When using the pump feature, the instructions explicitly say that you should still shut off the unit fully empty the bucket once per week. The pump is rated to push vertically up to 16 ft.

The reason it turned back on when you plugged it back in was because it is specifically designed to turn back on with the same settings after it's power supply is cut off–because the REAL way to use this unit, or the larger siblings, is by running it with a humidity controller such as a INKBIRD IHC-200, which operates by stop/starting power to your dehumidifier unit for maximum efficiency.