Mine has this problem too. It's a known issue and unfortunately hardware-related. There's plenty of threads with similar complaints if you search for them. Someone eventually made a "PSA style" open-letter over Redmi 5 Plus and it's touchscreen issues. An in-warranty replacement will sort it but if you're like me - bought on AliExpress and also out of warranty then you probably won't have much luck. I was still having this in-warranty but I didn't think it was worth attempting international repair with an AliExpress retailer. It's a shame, Xiaomi probably internally knows that a certain amount of Redmi5Plus stock is inherently flawed. I'd love them to swoop in and offer me an exchange but I don't think it's going to happen. I guess I kinda got screwed with this model.