Welcome to the forum!
First thing we need to check is the following…
- Right-click an empty space on the desktop and choose “Enter Edit Mode” from the menu.
- Right-click the desktop again and choose “Configure Desktop and Wallpaper”.
- Check whether Plasma is set to “Desktop” or “Folder View”. You need it to be the latter.
- Set the folder to view to be your Desktop folder — that’s where your icons live, normally.
If all of the above checks out and it still doesn’t work, then I invite you to come and commiserate with us on the threads below, where you may find a workaround — albeit that it’s a technical one, but the instructions are clear.
Long story short, Plasma 5.25.5 sucks and causes a lot of people to lose sleep. The workaround is to replace it with the still maintained Long-Term Support version of Plasma 5.24, which currently stands at version 5.24.7. This does however require creating a local repository on your own computer — again, the instructions are provided for, but more clarification can be added if needed.
The Plasma that’s currently in Manjaro Stable is at version 5.26.2, so we’re inquiring as to whether 5.26 fixes the problems that 5.25 introduced, because if it doesn’t, then we’re going to have to find a way to keep maintaining the 5.24.x LTS packages, given that those packages are no longer officially being offered by Manjaro itself — the reason why can be read in the first of the two threads I’ve listed above.