I recently put some new jungle Val in my tank and it looked awesome green at first and now it's just turning yellow an dying. They are putting off tons of runners and new Val but some are going yellow. Unfortunately can't upload pics to show...
My native collected vals did the exact same thing, if they are making runners and new plants then they like the conditions, sometimes the older leaves die when transplanting into water with different parameters, and sometimes they may just be older leaves, my vals grow so fast i trim with scissors otherwise they will grow to 5 feet long in my 75 gallon tank and take over the water surface, but once a leaf is around 6 months old they seem to die off naturally, i never notice, because the mollies and pleco eat them, or the malaysian trumpet snails pull them under the substrate and eat them.
I added about 25 or so to a 45 new tank.I had the same thing,a couple of them seemed to die off completely.But,they did send out new runners,and the resulting new plants grew like crazy,as did the other vals.
It's been an ongoing job picking out all the dead melted leaves,but every single one either sent out new runners and plants or new leaves.I don't think any of the leaves they came with lived.Fun watching them grow,and I don't think I permanently lost any of them.
They were also jungle Val.I hear they're really hard to kill.