SUMMARY
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Current Tag Tree allows breaking of tag hierarchy even when "Toggle-Auto - Parents" is on. 
if digiKam breaks tag hierarchy, then a sub-tag can be assigned to an image without its parents. Then any software that reads metadata from file will create a duplicate of the same tag outside its normal hierarchical tree. If the sub-tag is not unique, breaking of hierarchy can lead to data loss as without the context of the parent tags the sub-tag alone may be hard to distinguish.
***


STEPS TO REPRODUCE
1.  You need any 2 tags (A & B) with a common parent (C) in the Tag Tree (right side panel "Metadata" => "Tags")
2.   "Toggle-Auto - Parents" must be on (this option is already a must to avoid corruption of tag hierarchy as used in Microsoft programs such as OneDrive or WLPG)
3.  Assign Tag "A".  ==> digiKam will also correctly activate Tag "C".
4.  Assign Tag "B". ==> now the tags "A", "B" and "C" are all correctly selected.
5.  De-select Tag "A".  ==>

OBSERVED RESULT
5.  digiKam automatically de-selects the common Tag "C" and therefore breaks tag-hierarchy for the Tag "B"

EXPECTED RESULT
5.  the common Tag "C" must remain selected because there still is Tag "B" which needs "A" as a parent.



SOFTWARE/OS VERSIONS
Windows: 10

ADDITIONAL INFORMATION
https://userbase.kde.org/Digikam/Tutorials/Setup_of_digiKam_for_Windows_compatibility#Step_1_-_Enforce_Tag_Hierarchy