My bet is 6 gallons in an 18 gallon tank with the aftermarket sender and their inaccurate results its working fine.
I have tried 2 aftermarket senders and both read empty for the last 1/3 to 1/4 tank.
The fact that the gauge is rising is a normal part of thermo gauges and might indicate yours is functional. the IVR provides full battery voltage for a few seconds at startup to get the gauge and the IVR ready to work. once the thermo aspect of the IVR is working it will start turning on and off like a flasher unit providing the needed ABOUT 5-6 volts ( in this case a square wave). Real Time engineering MOPAR Products actually simulates this with their full electrical IVR.
If you want to see this in action, get an OEM IVR or the RT IVR and attach a small lamp to the output. when you first attach 12V to the input the lamp stays on full then after a few seconds starts flashing