You cannot add some materials from a library (.adsklib) into the project.
Also duplicating some materials does not work. Adding the materials by double clicking, dragging, using right click menu or using the arrow icons all do not work.
No errors or warnings, just not doing it.
The issue appeared to be in the material names. The materials that do not work contain square brackets "[" and "]" in the material name.
This issue did not happen in Revit 2019 (and earlier?).
Open the Material Library in 2019
Rename all the materials with square brackets.
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