It's my belief that you must be accidentally deleting them from your "Home Screen" as they call it.
What about swiping to the farthest right home screen until you get to where Apple kind of puts them into nice groups.
Can you find these missing icons there?
If so, press AND hold on an icon until they all starting squirming.
Keep a hold of it and drag the icon out. Keeping a hold of it, drag it clear to the left side of the screen and then then screen will pan to the left. Keep holding and dragging it until you reach the screen that you would like the icon to be on then let go of it. Be sure that there is an open space on that screen before you let go of it will create a folder.
Move apps and widgets on the Home Screen on iPhone - Apple Support
Here’s a video for you to watch ;-)
https://www.bing.com/videos/search?q=moving+apps+on+iphone&view=detail&mid=8E0BF1893813D7F75FE68E0BF1893813D7F75FE6&FORM=VIRE
Just remember to touch somewhere one the screen when you’re done and that will make them stop jiggling