Now that we have added subkey fingerprints to the details we should make it possible to copy them without white spaces somehow.
As you went for a separate button in T5776 for the main fpr, its likely that it would not be trivial to do this for regular copy/paste, right?
In the above mentioned ticket you decided against a context menu entry. Maybe this would be the best option in the case here? That is add "Copy Fingerprint" to the context menu in the subkey details tab.