SUMMARY
I have a workflow to create a jpeg reduced size copy of the files I have loaded in Digikam in raw (cr2) format.
Once the workflow has run, the thumbnails disappear. I have checked the file systems and the cr2 files are still in the designated folder. Also, the pictures can be seen if I select 'table' view.
Other thumbnails are still visible for folders where the workflow has not been run.

STEPS TO REPRODUCE
1. Select all files in folder
2. Run workflow (convert to jpeg, resize and save in different folder)
3. Thumbnails no longer appear

OBSERVED RESULT
Thumbnails missing

EXPECTED RESULT
Thumbnails still visible

SOFTWARE/OS VERSIONS
Windows: 
macOS: 
Linux/KDE Plasma: Gentoo 4.19.37, gcc 8.3.0
(available in About System)
KDE Plasma Version: 5.15.5
KDE Frameworks Version: 5.57.0
Qt Version: 5.12.3

ADDITIONAL INFORMATION
Worked fine in previous version of digikam