Now that I think about it, I wouldn't declare that Ladonna is leaving the show - yet.
It might be a similar situation as with the 1983 Charlie Brown special "Is This Goodbye, Charlie Brown?" where Linus and Lucy were moving away because of their father's job transfer, only to return at the end of the special, implying his father got laid off and returned to his old job.
The episode "When Duty Calls" might be considered a way to reboost viewership in order to continue producing episodes and have the show continue making money, since the show is losing popularity and ratings are in decline, which explains the episode order being reduced from ten to seven a few years ago.