Yesterday I noticed that the tallest stalk of my lucky bamboo randomly turned yellow. The foliage on top is still green. I have not changed its location nor water nor fertilizer; I thought it may have just been thirsty and gave it more water but it doesn't seem to be improving.





Thanks in advance for any & all advice.

I'm not sure but it may be rotting. I would dig it up out of its pot and check the roots.
By the way, 'Lucky Bamboo' is a misnomer, it is a type of Dracaena.

Look into chlorosis, often caused by increased PH from tap water over time, especially if the water has a lot of lime in it, and since it's a Dracaena, cumulative fluoride toxicity is a possibility.

That particular stem cutting has died. Once it starts to yellow at the bottom it spreads upward and eventually the foliage on top will die back as well. There is nothing you can do to save it. You can remove it at any time. This is not uncommon and hard to prevent altogether.

Something has caused the roots of that particular stem to die. That cause may also be affecting the other stems as well. My educated guess is that the soil is being kept too moist, particularly if there is no drain hole in the pot. Allow the top inch of soil to dry before adding any water and then add just enough so that it dries about again in about a week. Provide moderately bright light and warm temps.