The address to former presidents is usually "president (from-to)". But this rule has an important exception. When a former president becomes a party leader and/or a member of parliament, the use of "president" becomes completely inappropriate for the time in which he carries out such political activity. When he retires, he can then again be called "president (from-to). But while he is on the party field - categorically not.
This was commented on "Facebook" by the lecturer in constitutional law Assoc. Prof. Dr. Borislav Tsekov.
Let's respect the institutions and respect the constitutional principles. There is no way that there is "Iia the word the people's representative the president so-and-so". This is because in the National Assembly all deputies are of equal rank under the Constitution and their previous posts and titles, including academic degrees and positions, are completely irrelevant.
Even H.V. Simeon II, who bears the monarchical title for life and regardless of actually holding a monarchical position, because he was anointed by the Church as king, after his election as Prime Minister of the Republic, became "Mr. Prime Minister". That is why I have always addressed him in this way in an official capacity, and not with "Your Majesty"!
This is not a matter of protocol or personal preferences, but of higher constitutional principles. It is good to observe them if we want to be a civilized constitutional state, and not a primitive Balkan bantustan. This applies particularly strongly to the public and media space.