I noticed that if you rename a tag, changes appear in digikam but are not written to metadata, even when the option to write to files is checked in the configuration menu. Manually using the "Write information to metadata" in the main menu is currently required. This contrasts to other changes, like adding and removing tags, which are written successfully to pictures' metadata immediately. PS: it is possible that changing a tag hierarchy is also affected by this bug, since technically it's a form of renaming.
*** This bug has been marked as a duplicate of bug 370245 ***
Thousands of images could be affected when renaming a tag. This would take a long time. It is possible to use the maintenance tool to select only the new tag and have all metadata written. Or select the tag in the left sidebar, select all images and write the metadata in the images. An automatic update may be time critical. Maik
But the same applies with removing or adding a new tag. It's the user's choice if it's worth saving those changes to the pictures, with the time that it might take. If the option to write metadata to files is checked, it should be written. If it's not, only digikam's metadata should be changed. The current state is inconsistent, and because of that I just realized that many changes have not actually been applied to my library. A fair compromise would be showing a confirmation dialog before the change: "This change will affect ### pictures in your library, are you sure you want to continue?". (this is actually what happens in picasa if you try to write over more than 50 or 100 pictures at a time, if I recall correctly).
Fixed with bug #370245