The code was taking the last layer, but that is only valid if the mesh has only one kind of CustomData types (e.g. only UVMaps or only VertexColors). The solution I found is to call CustomData_get_named_layer_index instead. To avoid some situations where an uv with this name may already exist and the number of UVs is already the limit we are returning a CDL only when the texture is properly created. As a bonus that also fixes the same problem with VertexColor.