Since https://github.com/odoo/odoo/commit/4330ea02676a4afd88707bb463a007e7de3c8686, action_mark_won was not api.multi anymore, which made it impossible to mark leads as won in batch. The problem is that we do exactly this in the demo data. As we need to keep the rainbowman in the interface the decision has been taken to split the method in two parts, one for the interface which returns the rainbowman (and uses ensure_one) and the second which is a true api.multi method. Thanks to @tde-banana-odoo for his advice on the matter