Operating system: Windows 10 v1909
Graphics card: NVIDIA GeForce GTX 1660 super
The bug occured on the latest Steam release of Blender (2.90.1). I also tried to open the project file with the said bug in 2.83 LTS steam beta branch but it happened there too.
Short description of error
When trying to texture paint a certain project it ignores the "Sample Merged" setting and always picks the display output color, while other objects don't share this problem.
It happens regardless of the material or texture I'm editing, it seems to be tied to a single object. For example, If I create a cube and assign the material which the bugged object uses color picking works fine with it, and if I assign any other material to the bugged object it still samples display output color regardless of the settings.
Exact steps for others to reproduce the error
I'm sadly not sure how to reproduce it. Though I appended the bugged object to a fresh file:
It's system info:
I separated it so you wouldn't have to dig through the mess that is my main project file :D. It retained it's weird inabillity to pick colors though.
I couldn't find a solution to this when I tried to google for it, so my only assumption left here is it being a bug. I'm sorry in advance if something I said isn't clear. If needed, I can provide a whole project or try and provide more specific information if what I wrote here isn't enough as soon as possible.