Game concept
Movement Locked
The game also links it as Movement Lock.
From the game's encyclopedia
After an Army has completed more than half of a given move order, it will not be possible to change it until they arrive at their destination.
The game's text for patch 1.20.1.2. Words in blue are entries of the encyclopedia: hover one to read it, as in the game.
Movement Locked is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "After an Army has completed more than half of a given move order, it will not be possible to change it until they arrive at their destination." The game also links it as Movement Lock. Patch 1.20.1.2, from game/common/game_concepts/00_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/00_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.