With older routers (e.g. tl-wdr4300), I see a very significant difference (already pretty early in its life, while it was still my primary router) in brightness between the LEDs that were practically always-on (like power) and the less used ones. Especially the older blue (and slightly less so white-) LEDs do age significantly (and the tl-wdr4300 moved from originally blue to green LEDs in subsequent h/w revisions, maybe for cost reasons, maybe because of premature aging). Based on this, LED failure wouldn't be surprising to me at all.
Introduced to market in 2017 (and EOS around early 2022), so not even (possibly) that old…