S.I. Rosenberg, in The Atlantic, puts forth that Beethoven, just recovered from severe illness, might have created a new code, and a new set of dynamics, when he returned from illness to finish String Quartet No. 15, and the "Heiliger Dankegesang" movement. A violinist has done the research and makes the claim...

From the article:

Kitchen would eventually identify 23 degrees of dynamics (and counting), from fff—thunderous—to ppp—a whisper... Jeremy Yudkin, Lockwood’s co-director at the Center for Beethoven Studies, also initially viewed Kitchen with skepticism. “When I first talked to him, I thought he was nuts,” Yudkin told me. But Kitchen’s close and careful research won him over. Yudkin now believes that Kitchen has discovered a previously unknown layer of meaning in Beethoven’s manuscripts: “There are gradations of expression, a vast spectrum of expression, that music scholars and performers ought to take into account,” he said.

This article remains behind a paywall here...but if you look in Reddit, you might find enough of a synopsis to develop your own thoughts.