From my knowledge in tech, the problem usually comes from the part that is used when it comes to screen replacements. Some shops like to skimp when it comes to buying their stock and or take parts from other devices that aren't even compatible with the device that you have. Recently the same issue happened to me when I broke my Samsung Galaxy A15. Everything was working in order and I could feel how the phone still worked even if the screen was black and glitchy. Took it to get fixed at a shop and when I got it back, not only did the screen not even go to my phone model, but it also had this weird pink tint and a lag when I swiped. The quality of video's and things being displayed was also horrible. Replacing myself was not only cheaper, but it actually works like new and the lag is gone.