I’ve seen this happen too. Not always but it is irritating none the less. My solution is to drag the completed workout into the planned workout and merge them.
All, I hope that the hive mind can help me out here.
I have automated the sync between Garmin Connect and Training Peaks so when I do an activity it automatically shows up in TP, however it does not show up in the planned training box, but rather as a new session so for each day I have a box for planned and another for what was done. This is affecting my OCD tendencies and I wonder is there a quick fix to this, or do I gave to re word the actual session with the info from the planned one? and the delete the planned session from the calendar.