Repro from licensee here - [Link Removed]
Download the attached project "LayeredShaderBug.7z" and open in 4.26 editor.
Delete layer "Edge Wear", assert is thrown (see callstack)
Assertion failed: (Index >= 0) & (Index < ArrayNum) [Link Removed] [Line: 674]
Array index out of bounds: 3 from an array of size 3
UE4Editor.exe has triggered a breakpoint.
There's no existing public thread on this issue, so head over to AnswerHub just mention UE-114923 in the post.