Screen flickering above 1/4 to 1/2 brightness

[Covalence]

A week or two ago, I shut my laptop with a game open and left it on a blanket for ≈10 minutes, and a few seconds after I opened it up, the screen dimmed and started flickering. It did get a bit hot but it shouldn't have been excessive (81°C when I opened Task Manager a few seconds later). Now, the brightness control behaves like this:

• 0%: normal
• Somewhere between 28-50%: dims and starts flickering, dimming more as it goes up
• 100%: barely flickers, but is quite dim - visibly dimmer then 0%.

It's definitely a hardware issue as it happens everywhere, even the BIOS. There's also a buzzing sound (higher pitched with higher brightness) that occurs at the same time, around the top-left of the motherboard (near the CPU and display connector, but not centered on either of those). Some other things that are loosely correlated with it are:

• Power: unplugging/plugging in the charger at high brightness causes the screen to dim, then flash
• Heat: It works fine at half brightness when started up, but has to be turned down to 1/3 during/after gaming (70-75°C) or 1/4 on a stress test (85°C).

I tried reseating the display cable, and checking it for damage (nothing obvious), but it didn't change anything. Any ideas on how to fix the problem? The laptop is an HP 15z-eh000 (Ryzen 7 4700U, 16GB RAM, 500GB SSD, 1080p touchscreen).