im not sure why you would say "This issue was resolved long ago. Not sure why people are still posting about this but most likely because they don't have latest version of Windows 10?"
it was never resolved, people are posting about it because there is still the issue, (until version 1903)
how do you know? you seems to be assuming its fixed. just because 1 or 2 folders you tested are calculating the correct size, doesnt mean its fixed
i know this wasnt fixed in version 1809 because ive got a harddrive that is 1tb big and shows 4tb of files and folders (yes impossible) i did a clean install with 1803 and 1809, and it still showed the incorrect value, i tested on my other computers with 1809 and all 3 showed the wrong numbers. i did a clean install of windows 7 and it worked correct. microsoft only just released to the public 1903, i installed that and it finally works, so i dont know how you did your testing, but i know for a fact it wasnt fixed in 1809, also there feedback hub had a report from microsoft saying it will be fixed in 1903
this problem should now be fixed when you update to 1903, its a slow rollout though