@kucingbelang this sounds like a bad touchscreen. Of course, there is always a possibility that this is a bad touchscreen IC, but you want to replace the display assembly first. Since iFixit does not yet have a specific guide for this, Use [link|https://www.nadiemellamagallina.com/en/manuales/smartphones-1/oppo-703/oppo-a5-2020-1173/pantalla-completa-10391|this guide] to replace the display. It was written in Spanish so you may need to use a Browser that does the translating for you.
While you work on your phone, take lots of pictures. Once you are done with it, take those pictures and create a guide for iFixit. That will help the next person that has the same problem as you do. It's easy and fun to create these guides. https://ifixit.com/Guide/new The iFixit community will appreciate this.