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Try to change your MIUI. For example, if you are using MIUI global stable, try to change it first to MIUI global beta. If the problem is still persists on your device, unlock bootloader your phone then install custom rom.
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.