Of course. Tried refreshing, Firefox, Opera, Edge and Ungoogled Chromium to rule out possible browser issues. Just did a hard reset, re-setup the router, now we shall see what happens...
Edit: Seems everything is OK now. Re-enabled Traditional QOS, pie chart is working... My best guess is something got cocked up by enabling/disabling Trend Micro along with Traditional QOS. Also, I did not do a proper WPS hard reset after flashing Merlin. Just a Factory reset. We shall see how it goes from now on, but I am not toggling any Trend Micro related setting on the router, ever again. Asus did not need this in their firmware.