PROBLEM
Given two layers in a scene, a feature layer and a TIN, if you drape the feature layer on the TIN, set the feature's transparency, save and reopen the document, the TIN layer will end up with the same transparency as the features.
This is a known problem.
When setting the base surface for the features, specify the location of the TIN on disk rather than referencing the TIN layer in the scene.
Get help from ArcGIS experts
Download the Esri Support App