Altmark wasnt captured. She was boarded, and searched, and the british seamen held in her released, after the norwegians claimed to have serched her and found nothing....
Altmark was in breach of international law, by housing POWs in a non-combatabt that was also in neutral waters.
The Nrowegians were no happy at being caught out as nazi collaborators. Mind you the British Destroyer that undertook the operation was also flouting Norwgian neutrality