Hello, I think this is the old emersed growth dying off and is not a problem. Echinodorus are farmed out of the water in a humid atmosphere so they come to us with all emersed leaves. Then in the aquarium they grow new submersed leaves, and slowly the emersed ones die as they aren't as efficient for the plant under water as the submersed leaves are. It looks like the plant is growing lots of new leaves, so no worries!
Perfect explanation above. They grow their new leaves from the centre and it's these leaves that are the best indication of the plants health. As these tend to be vigorous plants you often get older leaves looking a little tatty, so just nip them off at the base. Echinodoras are heavy root feeders so popping root tabs around them regularly is ideal.
As far as I can tell, there must be something wrong with your plant. It can either be light, the absence of fertilizers, or both of these factors, or it could be something else entirely. You can take some remedial measures in this regard if you wish to do so. As a first step, you should remove the yellowish or brown leaves from the plant because they have already started to deteriorate, and rebuilding them will require a lot of nutrients and energy. There is no doubt that a root tab is one of the most important things to have in the tank, especially if you have that type of Amazon Sword or other aquatic plants in the tank. Another way to revive yellowish and brown leaves is by injecting them with carbon dioxide, which is another solution to revive them. If your fish are exposed to too much carbon dioxide, they might die as a result. I would appreciate it if you could do so accordingly, and I hope that would work for you.