It is exactly the issue discussed in the linked issue.
Since the Android 12 update (which was in February), the FP4 has a cooling feature that reduces the maximum brightness even when the slider is all the way to the right.
This occurs when the phone heats up to ~40 degrees at the CPU, which is not a lot at all.
So yeah, that’s the issue. FP said, a fix is to be expected in October. Until then, feel free to use my workaround if you are rooted.