Some software, including Digikam, use / as a separator for hierarchical tags. Some other software (for instance darktable) use | instead.
That breaks interop and the ability to use both software on the same collection of files.
To solve this, Digikam should recognize both / and | as separators in tag hierarchies.
If there’s a concern about pre-existing content using | and expecting it to be treated literally, that could be made into an off-by-default option, though I am hopeful that the fact it is a rare character in general, and that other software already use | as a separator, will mean that it is rarely used literally, and we can have this always on.
Also, an option to chose which of the two Digikam should use when adding tags would be maximize the ability not just to import from, but to work with other software on the same collection of files. I think this can be a global setting. A more fine-grained version of this could be a per-tag setting, but that seems overkill.