This could mean that something isn’t assembled correctly inside of the phone and is pressing against your display panel. You can easily test this by opening the phone again and to not press it into the case. If you still see white dots it probably means that your display got damaged during installation or that it was damaged before installation.
I noticed something similar on an iPhone 7 Plus I worked on where the display flex cable was not correctly routed under the shield of the screen, causing some pressure on the screen resulting in similar white dots when touching on the display. Take a look at the screen replacement guide as of step 34 to double check your cable is routed properly. I’m saying this because the white spots in your pictures happen to match the placement of the ribbon cables.
Did you test the screen before fully assembling the phone again?