Wiktionary
methylase
n. (context enzyme English) Any of several enzymes that catalyze methylation, especially of nucleic acids
Usage examples of "methylase".
But maintenance methylase would do exactly the same thing to it, duplicating cytosine methylation on both sides, if originally present on one side.
Maybe the demethylation is due to cumulative damage to the DNA itself, maybe the methylase enzyme becomes less active in the course of time, maybe something else.