This has happened to me several times and it’s easy to reproduce
1- I open the file FIRST.TIF (previously saved with the “save and edit later” option active) in Nik collection Color Efex Pro (Export setting “export selected files without processing”);
2- I do all the editing and I save the image withe the “save and edit later” option ticked, saving procedure starts;
3- Meanwhile I switch to Photolab 3 and I open in the same plugin the file SECOND.TIF previously saved with the “save and edit later” option active (Export setting “export selected files without processing”);
5- Nothing stops me to do this, so the SECOND.TIF file results opened in the Color efex pro window while it is saving the FIRST.TIF file (two open files are visible in the bottom status bar)
6- At the end of the saving procedure FIRST.TIF file is written properly, SECOND.TIF results damaged and is unrecoverable. In its place there is a second copy, I guess the editable second layer, of the FIRST.TIF image. If I watch it in Photolab I can still see the SECOND.TIF image but if I try to open it in the plugin to carry on the editing I couldn’t do before I get the FIRST.TIF image instead of the SECOND.TIF which has disappeared.
7- Restart editing SECOND.TIF image from the RAW.
It would help to have an alert message in a pop-up window that blocks the user from opening a file while the plugin is still working on the other one.
Hope this helps.