Why is my portable air conditioning unit increasing relative humidity?
If the A/C unit is cooling the room but not running long enough to remove the humidity, then YES, the % of relative humidity will rise. If by chance the A/C unit is too large for the space it is cooling then it will not run long enough to reduce the relative humidity.
Also, those type units exhaust air from the room as it is running and unconditioned air has to be migrating to that room to make up for the air that is being expelled. A better choice would be a portable unit that has 2 pipes; 1 to bring in the required air for the unit to operate properly and one to exhaust the warmed air that is produced by the units operation. I call the units with 1 hose a 1/2 A/C unit since they only satisfy 1/2 of the air requirements for proper operation. For a more comfortable room you need to install the correct unit.
To better understand humidity, you need to get a book or search online and learn about the relationship between % of relative humidity and air temperatures. A quick learn is that as air temperatures fall the % of relative  humidity rises and visa-versa.