Histoircal battle "Bapheus (1302)" in campaign

When I played it, I got the achievement with no problem back in April 2020. (In response to OP:) You have to read the wording of the Objectives very carefully. It’s very clear that the scenario ends if you destroy 3 byzantine castles, and the defeats of the other factions are optional objectives; there is no bug there. Therefore you must defeat all other players before finishing off the last castle.

Like with any long scenario facing an achievement, I suggest saving often when doing it.

Although I did this in April 2020, @AngrySmores , there could be a bug now with the steam achievement.

Edit - It looks like Smoke on the Water Achievement (Lake Poyang) Not Firing is related.