Abbasid Mastery has several missing resource text values for each of the Test of Strength entries. (“win a 1v1 custom or skirmish match against the ___ AI”) It is showing these 4 items that should have “Test of Strength” text:
This is actually showing up for me on Chinese mastery as well. The other masteries show up normally.
Just open the Masteries screen and click into Abbasid. The missing resources are visible there, or as a title if you click on one of them (such as mastery_abbasid_15).
I recently completed the Chinese mastery and I noticed this issue shortly after. I had previously completed these masteries: English, French, Holy Roman Empire, Mongol, Rus, Training. I have not completed the Abbasid nor Delhi masteries.