"morphologically unchanged" seems like a very low bar for lichen too. Are we talking like electron microscope slides that compare their microstructure? It seems like fossilized lichen will have a lot of morphological overlap between many species, possibly ones that aren't even lichen.
You'd be surprised what they can find these days with an electron microscope and a sufficiently well-preserved fossil. Microstructure is sometimes visible. But yeah, there's still only so much you can do.