Why is my screen flickering when the brightness is turned down?

Hold the D key down and press the power button, and see if happens there. That will boot the laptop into a isolated test mode to allow you to isolate it to hardware or software. If it flickers there, it’s a hardware or power issue. If it does not, then there’s a software issue. Some of the Dell LED displays do not being like the absolute bare minimum setting and at least need to be at the minimum +1 for stability. LED is more sensitive to undervolting as a whole - it usually doesn’t kill the array but weird issues like this come up. In most cases where it’s isolated at the OS level, the display is dim enough you notice the refresh rate and there’s no fault with the display.

A lot of the 12” systems (E6/E7/52/72) split the load between the battery and power adapter since you can’t cram a barely TSA legal 95Wh in a 12” ultraportable. On some of them it’s so bad you need to compensate with a bigger adapter (Ex: factory spec 65W, but can use a 90W as a way to allow the machine to run at full speed with no/weak batteries). The reason it sometimes helps is if it's cutting back due to a limited range of available power is it can call for more to pick up where it needs the help. If possible, stick to 90W adapters when possible.

With yours being a Core M device, the power issue may be less of an issue compared to a machine with an i5 U chip for example, since the Core M series is meant for very low power devices. It may not even rely on throttling as much as the normal devices - if at all. But yeah, it’s a known thing with these subcompact notebooks to split the power load between the adapter/battery or reduce the available power heavily.

Sometimes replacing the battery does help with these issues, especially if it’s a few years old. However, it looks like a mildly difficult swap :-(.