Found a solution!! Choose “Stop Managing Screen Time” on my device for the individual. And then set up Screen Time again, like you’re setting it up for the first time. This was the only thing that worked for me and will hopefully work for others!
I had the same problem as others stated: I had stopped allowing App Store purchases, but after allowing again, the App Store wouldn’t show up on my child’s phone. Or, it would show up for a second and then disappear. I tried turning off Content and Privacy Restrictions, and toggling it back on again, but this didn’t help. We backed up her phone, reset it and erased all settings, then set up up again from the backup, and the problem persisted. We even erased/reset it again, this time not using her backup, and once I connected her to iCloud and had my screen time controls connected to her phone, the App Store disappeared again. Finically I chose “Stop Managing ___ Screen Time” and set up the screen time controls fresh and it worked! So it must have been some glitch that was embedded in the way the controls were set up and those controls needed to be reset by stopping management of them and then starting again.