Sounds like Stena are following P&O's lead.
"In advance of Britain leaving the European Union on March 29 2019, we undertook a review of the flag status of our ships on the English Channel... For operational and accounting reasons, we have concluded that the best course of action is to reflag all ships to be under the Cyprus flag.... We have no plans to make any other changes, including the terms and conditions of any of our seafarers, as a result of the new arrangements." - P&O Ferries