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Second green row in my provided link:
"Fixed in 2019.4, 2020.2, 2021.1"
https://issuetracker.unity3d.com/issues/package-manager-doesnt-show-available-updates is saying issue is fixed in this version....
I think hbonin said, bug fix in the review and not published yet.
As far as I understand, first part of the message was describing how it should...
Looks like this problem exists for few weeks.
There are another thread for it...