In one of the two Fonts of Secrecy, one of the 6 required seals seems to be missing to unlock all of the pylons (in my game, it was the one without the water pools).
I've scoured each surface of this puzzle for at least an hour over 2 days looking - so it's either the most well-hidden seal in all of Nightingale or it's missing and now blocking my progression down the Moor path.
Looking around online, I was able to find 1 youtuber that had videos out of his two Secrecy puzzles but the only one he covered was the one I was able to complete without issue.
Follow up: after exiting the game for a few hours and returning, I was able to see the missing sigil. In an area I'd looked in before (it's possible that it didn't light up red like it's supposed to when you get close to it, and thus missed it previously)