I want to be sure I understand this. Are you talking about Photoshop CC 2018.app being on the desktop or a desktop shortcut to Photoshop CC 2018.app inside the /Applications/Adobe Photoshop CC 2018 folder? The .app belongs in the folder, not on the desktop.
My strategy is to launch Photoshop from the app folder and when I see the icon in the dock, ctrl + click on it and choose "keep in dock" so the shortcut stays there.
As an extra, run First Aid from Disk Utility to be sure nothing is wrong with your boot drive.
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I think the problem are corrupted shortcuts. Please try to rebuild the shortcuts.
If there is a problem with the shortcut I've found this hint on a forum:
"I was able go get a phone help desk session with Apple. They connected to my screen to see what was happening. They discovered the items in question were simply shortcuts that were not working. The actual folders in question were working just fine. They did have a bit of a struggle to get the shortcuts removed however. What finally worked was to hold Command while dragging the icon to the desktop. They simply disappeared when doing that. Problem solved."