I am not sure that if this is a Reaper issue or Ultraschall issue, but I haven’t found anyone mention it in Reaper community.
I usually work with multiple projects simultaneously, sometimes switch between .rpps, sometimes open multiple project tabs together. I have been noticing that sometimes source files associate with project A (shown in project bay) will show in another project B for no reason, I’m sure I have never inserted these file to the project B in any way.
This actually happens quite often, sometimes when one project is finished and the file was move to somewhere else it would cause „missing source file“ dialogue when opening other projects, and slow down loading speed as extra unused sources are loaded.
In the project bay, shown as (un)availiable. They are not actually active in any track therefore only visible in the project bay, but they would be loaded, so I will get „missing source file“ error if the files are not available.
I do not use any extra project template, only the default Ultraschall one, I modified it a bit, removed the default track FXs, there’s no track nor associated source file in the default template.
Since Ultraschall can’t really program the project bay, it’s either a bug in Reaper or leftovers from deleted items in the project.
The latter might leave the sources still in the project bay occasionally, though I never tried to find out when this happens and why.
The best way to deal with it is probably deleting the missing sources from the projectbay and save the project again.