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.
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.