Answer
Only the FND_WEBRUNTIME and FND_RUNTIME permission sets can actually be connected to an LTU license — if you're granting an additional permission set (like one needed for lobby/projection access) beyond what your LTU's license file was originally generated to cover, you need to have that LTU's permission-set configuration revised and the license file regenerated to match, or the license itself becomes invalidated rather than the user quietly consuming a Full license. This isn't something to change casually on your own — contact IFS Consulting to have the license file regenerated based on the revised permission set whenever you add new permissions intended for LTU-licensed users.