There is a problem with your backlight circuit on the board itself.
In itself it is not a big issue. However, there are complications ahead.
First, have you had any touch issues previously? If not, for 6+ there is some risk of developing one after the backlight issue is repaired. Sometimes heating can trigger this "waiting to happen" fault.
Second, the backlight circuit is just around the CPU and that's where the second complication lies: one has to be conservative when trying to repair the backlight in this area to avoid harm to the CPU. Avoiding damage to the CPU is what limits the repairability to experienced techs.
The cause of this is that a component on one of the two b/l circuits that this phone has failed. Could be an anode, cathode, b/l driver, or diode.
My advice is to live with it if your only option is DYI because it's not a DYI repair.
Otherwise if you're ready to spend some money to fix it, take/send it to someone with enough experience in this type of repair.
If you don't know anyone reliable, check my profile here on iFixit for a link to a website that has a list or tried technicians. From those, if you're in or near the US, I suggest you use ipadrehab. If you're down under or near Australia, you have geelongmicrosoldering.