My comments are based upon my 40 years of horticulture
My Comments are based on 30 years of growing crypts.
What the hell has root uptake got to do with a movie franchise?
Because just like in the movie, it seems you have been programmed to think a certain thought that has nothing to do with reality.
Now, I have no idea what branch of horticulture your expertise is in but either your statement regarding "crypts needing to grow roots and not needing CO2 at that stage" is mistyped or your horticultural expertise is in an area far removed from the science of the metabolism of aquatic plants. In fact, there is no way that any plant, whether terrestrial or aquatic will choose to grow roots as a priority over gas exchange. Plants cannot even grow roots without first assimilating CO2 because plant roots, like plant stems, plant flowers, plant fruits and plant leaves are physically constructed primarily of carbon. In Northumberland there are gigantic mines filled with coal. This coal started out as the bark and tissue of trees. Coal is carbon. That should offer a clue as to the importance of carbon to plants. The petrol you fill your tank with and oil used to heat your home is hydrocarbon and is a descended from the carbon contained in the trees of ancient forests. Over 50% of a plants dry mass is carbon. One of the main reasons we have global warming is specifically the fact that we have cut down so many trees that the forests of the world are now unable to remove sufficient quantities of CO2, which lingers in the atmosphere. That's WHY CO2 is called a greenhouse gas.
So Crypts don't melt for any other reason than not having a co2 system
Err, yes! ANY structural flaw in aquatic plants while underwater is caused by poor CO2. That includes: deformation, holes not otherwise caused by predation, translucency, browning, decay, black spots as well as certain forms of algae such as green filamentous, hair for example and red, such as BBA.
For 30 years I've had to listen to people bitching and moaning about their crypts melting, and how it's caused by moving the plant, or by changing the water from soft to hard, or from hard to soft, or by not keeping the water parameters constant, or due to nutrient toxicity. All the time, my crypts never melted no matter what I did trying to replicate these boneheaded theories. I could not get crypts to melt until I reduced the CO2, and viola, melt. This is repeatable and is consistent. Everyone thinks their CO2 is excellent. I NEVER think my CO2 is excellent because I know it is the most important and the most difficult aspect of aquatic husbandry.
If your crypts melt - even if the leaves grow back, then that is an early warning that your CO2 is suspect and that you probably will have issues in the future.
As Barr (plantbrain) says, you must be willing to destroy your tank in order to learn the truth and you must be able to grow plants problem-free before you can ever hope to determine why they fail.
I've modified his postulate to wit: you must be willing to destroy your tank in order to save it (see Apocalypse Now).
Cheers,