I will paste below the post I have already sent to the Edge 1040 forum, because the same is happening when on a bike Workout on the forerunner 955. The difference is that you pause an activity during a bike Workout and then select resume later. After starting again the activity, he workout timer has usually progressed or finished and the activity timer is about 50 minutes to 1 hour more. During a simple 30 minutes recovery workout I paused with about 12 minutes remaining and when restarted the workout was finished and the activity timer was sowing 1 hour and some minutes. I'm on the latest stable firmware on both devices.
Manual pause and standby during a workout, results in either time jump forward of the workout and the activity timer as well or workout timer stuck and activity timer unaffected. ex. I paused the Edge and then pressed the power briefly to standby during a 2 hour daily suggested workout with only one step, after a few seconds of the workout had passed and when I woke it up after 10 seconds and pressed start (right bottom button) the activity timer added 2 hours, and the workout timer jumped to 0 time remaining and the workout ended.
The issue above is affecting the forerunner 955 which show's that this bug is probably a more fundamental issue of how the workouts coding nests in the rest of the firmware