I can't say what is wrong but I can help narrow down... These are just some tips in helping to diagnose plant problems.
Since the brand new leaves are not yellowing, you know that whatever is wrong is not [likely] due to a nutrient deficiency. (Nute deficiency can also be caused by over fertilizing, the roots 'lock up' and won't take in new nutes in causing symptoms that look the same as a deficiency). Over or under fertilizing can show as burning leaves or the brand new growth yellowing. Severely deficient plants can show problems on old and new growth, so the timing that you recognize something is important to consider also.
New growth and old growth react differently to problems. So a good thing to always consider is where the problem is showing itself. If it is new growth showing problems then whatever is happening in the soil right now (or a worm eating the stem from the inside out) is the cause.
However, if there was a problem with the lower leaves, and the new growth is lush and green and happy, then whatever problem occurred, likely occurred in the past and you have already fixed it. This is important because a lot of people OVER CARE for their plants. And over caring can turn into other problems.
Typically, if the lower leaves are dying but the new growth seems fine, the plant will also be fine and is [likely] not a lack of nitrogen, but there may be other micro nutrients that are missing. OR, and this is a big one, the PH may be way off. Or, there could be watering issues. OR..... well you can see how it can be very hard to diagnose (correctly) problems on a plant.
So, sorry I don't know what is wrong, but hopefully something I mentioned helps in the future.
Oh, and sorry for this super confusing post. If it actually makes sense to you, then you already know what I am talking about, or have a good future in diagnosing plants.
disclaimer: the things I mentioned above are by no means "rules." They are just a few of many different possibilities. But good to know IMO. If anyone sees anything I said as incorrect, feel free to correct me, it might only help new diagnosticians. And my post isn't super comprehensive considering the many things that can be... but I know that I have brought many problem plants back to great health, so I know I understand what I do, but it is pretty tough to describe in writing.