After exceeding a (large) number of notes on the map, only the most recent are listed or showing anything when selecting them. The rest show the flashing letters, but no info, can’t be edited or deleted. Though they can be overwritten with new notes.
It seems like you’re right… I started a new game with a flat (all 9s) character and found a big wheel lying some hundred yards off a military bunker. Made a note, now it shows txt but there is unfortunately no blinking.