That vertical line on the right is what the Fill Column Indicator package is inserting. With that persistently enabled, you can see that it messes up the beacon.. the cursor keeps on jumping to the set column.
I tried this to disable fci at the right time:
(defun beacon--shine ()
"Shine a beacon at point."
(setq-local sanityinc/fci-mode-suppressed fci-mode)
(when fci-mode
(turn-off-fci-mode))
(let ((colors (beacon--color-range)))
(save-excursion
(while colors
(if (looking-at "$")
(progn
(beacon--after-string-overlay colors)
(setq colors nil))
(beacon--colored-overlay (pop colors))
(forward-char 1))))))
But I cannot figure out where to reenable it.. it didn't work if I tried to renable at the end of beacon--shine or beacon-blink.