Remember I haven't needed this since about 2010 to 2015 and I don't recall TPL back then. TPL claims to scale and the threading in .NET/C# wasn't that capable. It did thread but only on a single core of the CPU. So you had to be creative to tap all the cores.
Try this. See if threads now spread out among the cores. If they do, Microsoft fixed that. And then test in TPL and see if TPL was their solution.
Maybe it's all in that one word "scaling."