The diplomacy dialog still displays enemies as neutral since rP19247, because the removed updateDiplomacy() in gui/common/ updated the playerdata too when the ceasefire ended.
I confused the old updateDiplomacy with the new updateDiplomacy when reviewing. I guess one of us should have tested the diff.
The proposed patch adds a new GUI notification that is sent if the ceasefire has ended, so that we and modders could do some arbitrary actions if the ceasefire has ended.
Calling updatePlayerData onSimulationUpdate would work too, but readers would not understand that it's currently only due to the ceasefire in that place when not adding a comment.
Notice there is no race condition, as first all notifications are processed in onSimulationUpdate by calling handleNotifications (diplo, ceasefire, defeat, win),
after that the diplo dialog is updated by calling updateGUIObjects.
So for all current updatePlayerData calls an updateGUIObjects follows.