When you add the Weight Training workout to the Planner it is tagged as Planned.
That means it’s holding a place on the calendar with the expectation you will perform it at the specified start time.
Unlike other workouts the weight workout isn’t something you will actually run on EBC or export to run elsewhere. It’s being used to simulate high/peak strain on the Planner so the form calculation gradient applies it and shows you as tired for a period afterwards.
Since the workout isn’t actually performed it disappears after the scheduled start time + duration. When it disappears the form gradient changes to reflect the fact the planned activity was never completed. So, the solution is to add it back with a date/time in the past to indicate you did indeed complete it. This can be done on the same day by making sure the start time + duration is earlier than now (completed in the past).
The Weight Training workout was added to the library because many users wanted to include the strain/stress from their gym work. Something that affects their cycling the next day or so. I don’t use it because my strength training is limited to bands, bodyweight, and a kettlebell. I don’t do anything strenuous enough to hinder my pedaling the next day. YMMV