There doesn’t seem to be any inherent limitation on Shopify’s end that prevents self-serve returns for B2B customers. Can you DM me the store where this is happening?
After looking into this a bit more I’ve discovered that in fact self serve returns do not support B2B orders - which is why you’re not seeing an option in the order to trigger a self serve return. Apologies for my earlier answer that indicated that it was supported - I’ve forwarded your feedback to the team who is working on this however!