Dunno which era that one is, my XPS15 9560 never dims at all, it's always bright, maybe that's because I'm missing some sort of userland or kernel driver, but frankly I like it this way.
It's an issue across generations (and even across manufacturers, it's the displays firmware after all). I think it's the 13" FHD version affected. My 15" is also fine.
+1 to this, currently typing from a two-weeks new XPS15 9560. Manually installed Windows and graphics drivers, I have tinkered with the Intel HD Graphics settings a fair bit previously, however. Would not be surprised if that were the culprit to some extent.