TestActor references Shape_Cube_2 and Shape_Cube_3.
TestActor blueprint shows the logic
2DSideScrollerCharacter blueprint contains the keyboard control logic.
Download the licensee repro MaterialTest - [Link Removed]
Open in 4.26, hit Play
Pressing C will switch./spawn the mesh referenced visually by "TestActor". One is blue, one is red.
Press X, the current mesh will have it's MaterialInstance cleared to none/black.
Press C again, the meshes still switch but they all are black, the material instances have been cleared from both meshes.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-115079 in the post.