Planned activities will expire based on the time scheduled but never performed.
By default the As Of time under Goals is set to 11:59pm which is also the default time for a workout selected or activity dragged to the Planner. In this case the activity will disappear by the next day but you could also edit the time for the activity added to the Planner so it will expire earlier in the day.
Related – Planned Workout Disappears? - Support - Xert Community Forum (xertonline.com)
Welcome aboard. 