Power curve shows approximately half actual power

Not sure whether it applies to your case, sice you did not post the link to the activity, so we cannot have a look at it, but one possible reason is described here: Viewing Power Curve in Garmin Connect | Garmin Customer Support (see the section "My Activity Has a Higher Average Power, Why Doesn’t It Show on the Power Curve?")

Some activities have a higher overall average power for a split, or total activity, compared to what is represented on your Power Curve. This is because Power Curve looks at the elapsed time of the activity, if Auto Paused is enabled you may see instances where the are differences between power numbers in an activity compared to reported power on the Power Curve.

For example, if you ride for one hour at a constant 200 watts but have 5 minutes of Auto Pause time in the last 30 minutes of the activity, those 5 minutes are removed from the activity time but are still included in elapsed time. The activity’s average power will be 200 watts, but the average one hour power would be 183 watts on the Power Curve because Power Curve uses elapsed time for the activity.

Another possibility is that the data from the external app comes in a format not entirely compatible with Garmin Connect.