This is not something that can be easily fixed.
The new custom Quickbar would have to show wherever a mobile player has placed their normal Quickbar.
Any kind of difference in the default padding also extends the Quickbar, which can lead to the custom quickbar blocking more than it should.
A very large amount of mobile players also do not use the normal quickbar. They use individual slots that are not attached to each other. A custom quickbar would not work for this.
The only solution I can think of for this is to actually remove the custom quickbar from touch input on mobile entirely. Epic could work on adding custom Quickbars that are built for mobile only. Every creator would need to make one custom quickbar for mobile (touch input only) and then another for the other platforms/inputs. There is just no way a quickbar designed for the controller/KBM HUD will work well with mobile.