Upon opening the advancements area, the background for each area (Nether, Story, etc.) is solid black instead of its usual background (Netherrack, Stone, etc.) I looked under changes for 1.15.1 (affected version) and saw that this is not intentional, therefor it is a bug. To 'reproduce' this bug, all one has to do is run the launcher, when Minecraft has fully loaded, create a new world and gain an advancement (ex: Stone Age). Then go to the advancements area, and once its open, you should immediately notice the background is solid black instead of what its suppose to be (ex: Stone background). 

Issue is NOT tied to any resource pack. Tested with both default textures and Programmer Art. Neither are the source of the issue. Sorry for the mishap.