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There may be more than one thing causing these signs of disorder. I live in Thailand and with the current wet season it is hard to keep plants pest and disease free.
The dried discolored drying/dying growth could be from a fungal disease. And the new leaf distorted growth is probably a sucking insect pest on the under side of the leaves, like aphids or psyllids or leaf hoppers. And sometimes it is the pest that vectors/introduces the pathogen into the plant tissue.
All you can do is to keep the plants as healthy as possible with good water management and fertilization.
I don't know your resources there in Malaysia, but here in Thailand the large shops like TESCO and HOMEPRO have some gardening proucts, and we have Lazada. I see that Lazada is in Malaysia too, and they have a neem oil product available that can be used for pest suppression. You will need a small spray bottle. https://www.lazada.com.my/products/organic-verdana-cold-pressed-neem-oil-32-fluid-ounce-un-refined-filtered-high-azadirachtin-content-for-indoor-and-outdoor-plant-spray-plant-care-pet-care-skin-care-hair-care-i145050411-s169008783.html?spm=a2o4k.searchlist.list.19.356a5027Q68uel&search=1

This Black Gold organic fertilizer product looks good,but I haven't use it. It has a trichoderma biological fungicide component too. This can be sprayed and applied as a soil drench.
https://www.lazada.com.my/products/blackgold-bio-organic-fertilizer-with-trichopower-mix-greenearth-re-packing-1-kg-i360566918-s511144644.html?spm=a2o4k.searchlist.list.5.513e7cd5EwuDOD&search=1

It can be a bit overwhelming at first, but they will be somewhat tolerant of mistakes. I have found an article for you that will give you a little more specific detail. If you are ever in doubt, you can just grow then in container and keep them as you would a standard houseplant. Otherwise, follow these instructions from this article: https://www.gardeningknowhow.com/ornamental/bulbs/dahlia/dahlias-annual-or-perennial.htm

If you generally leave in the ground in your garden, you can leave them.

If you want to dig up, I would wait until it gets colder or the foliage gets touched by the cold.

https://www.gardeningknowhow.com/ornamental/bulbs/dahlia/storing-dahlia-tubers.htm

Leave roots intact if they are plump. The very fine roots can be removed but this isn't necessary. Here are articles; the first link is on storing dahlia tubers and the second is on identifying the "mother". You might enjoy browsing other pages on the 'dahilas.net" website. https://www.gardeningknowhow.com/ornamental/bulbs/dahlia/dividing-dahlia-tubers.htm
http://www.dahlias.net/dahwebpg/TuberStor/TuberStor3.htm

This article will help you.
https://www.gardeningknowhow.com/ornamental/bulbs/dahlia/grow-dahlias-in-containers.htm

If they're soft, you've lost them. If they're firm then when it comes to potting them up soak them in warm, water for 24hrs before potting them.