The usual problem I get with Rex types is that stem gets too long and snakes across the pot rather than having a nice compact clump of leaves. They don't grow a tall stem. Is there a reason you want a long stem?

Rexes (a form of rhizomatous) are supposed to be short stemmed. This is an unusual growth pattern on your plant as though it has been staked to grow vertically instead of its natural form of bushy.

It should look more like these.

Oh sorry. I mean petiole.
The petiole are so short that leaves overlap and grow into crumpled and strange shapes. I've grown a lot of Rex, but I've never seen anything like this before.

could be a mutation from chemicals or some pathogens

perhaps the leaves that you see are not from the tip (older larger leaves), but from new buds on the stem. On most of my larger leaved rexes when new growth starts from the side bud the leaves stay quite small, until you have 3-4 leaves. then they get bigger.