Workout data can be synced only one time from Mi Band 4 (due to watch firmware limitations).
When also official app is installed you have to follow the correct procedure to be able get workouts synced on both apps, otherwise workouts may get synced only on one app (the first one to sync the data).
This is Mi Band 4 firmware limitation, not related to Notify app.
If you keep both app opened (background) and connected correctly (before start sync new data from Mi Band 4), you should be able to obtain data on both apps. Start official app firstly to sync workout data and then new data should be available also on Notify app automatically.
You cannot resync anymore a workout once official app has synced it the first time.
Only sync one workout at once to avoid additional conflicts with official app installed.
Remove official app to avoid this kind of sync conflicts.
GPS data missing on Notify but available on official app?
Check the workout settings on Notify app