Created attachment 144884 [details]
Adding the tag works, but filtering by them doesn't.

SUMMARY
When adding tags to an image they don't "register" in the program. The image itself gets the tags added, but gwenview doesn't allow me to filter images by tag, and the drop downs for selecting tags are empty.

STEPS TO REPRODUCE
1. Open image in gwenview.
2. On the Information tab of the side panel, next to "Tags:" click the "Edit" link.
3. Type in the name of the tag (e.g. "test"), click the "+" button, and close the dialog.
4. Click on the "Browse" button to display all images in the current folder.
5. Click on "Add Filter" and select "Filter by Tag".
6. In the filter, select "Tagged".

OBSERVED RESULT
Notice that no tags are available to be selected in the dropdown, and typing in the tag name doesn't do anything.

EXPECTED RESULT
The dropdown would offer available tags that were created previously to be selected and filtered by, and they would be searchable by typing.

SOFTWARE/OS VERSIONS
Windows: N/A
macOS: N/A
Linux/KDE Plasma: Linux 5.15.11-1-MANJARO / KDE Plasma 5.23.4
(available in About System)
KDE Plasma Version: 5.23.4
KDE Frameworks Version: 5.89.0
Qt Version: 5.15.2

ADDITIONAL INFORMATION
Tags added through gwenview can be seen by dolphin.
Adding tags from dolphin has the same behavior.
After some time the tags went away, possibly due to software updates.
Other filtering types work.