It’s likely that your wall-wart doesn’t provide enough watts to fully charge your iPad mini, and/or that there’s some reason the USB-A side of that cable isn’t adequate for what the iPad mini needs.
If you want to test, consider trying with a non-Apple wall-wart for which the rated wattage is equal to or greater than the one which Apple provides with your iPad mini and which uses a USB-C connection rather than a USB-A one. If it comes with a USB-C to USB-C cable, use that, otherwise get one that supports USB-C PD and enough watts to match the iPad mini’s needs.
That's not fully true, and even if it's partially true in some cases (this depends on the chemistry of the battery): volts and watts aren't the same thing. You can be fully capable of supplying 5v@2.4A and not capable of supplying 12v@1A which are the same number of watts.
Battery tech is a horrible black hole that is not very fun to dig into, chargers are a little bit more transparent: with markings for various voltages and amperages printed on the device.
iPad batteries output 3.7v if I'm not mistaken, but I'm unsure what they charge with.
A 3.7V nominal li-ion battery would peak at about 4.5V while charging. A bit high, but a well designed circuit should be able to do that off 5V. Besides, 75% is far short of where the voltage starts to spike.
>volts and watts aren't the same thing. You can be fully capable of supplying 5v@2.4A and not capable of supplying 12v@1A which are the same number of watts.
For the layman, the equation is Volts x Amperes = Watts.
Where if we use the common water examples: Voltage is electric charge ("water pressure" or "volume of water"), Amperage is electric current ("water flow rate"), and Wattage is electrical energy ("amount of water transferred").
2V x 6A, 4V x 3A, 1V x 12A, 12V x 1A and similar are all 12W but they are obviously very different in nature.
If you want to test, consider trying with a non-Apple wall-wart for which the rated wattage is equal to or greater than the one which Apple provides with your iPad mini and which uses a USB-C connection rather than a USB-A one. If it comes with a USB-C to USB-C cable, use that, otherwise get one that supports USB-C PD and enough watts to match the iPad mini’s needs.