Elevation Gain Data inconsistent between Strava and Zwift

The Auto Upload feature from zwift to strava shows incorrect elevation gain in the TCX file exported from Strava - zwift data for the Biggest Loop Watopia Loop shows elevation gain of 704mts whereas the TCX exported from Strava shows 1677mts.
On the dashboard Strava shows Elevation gain as 0mts.

Why the inconsistency? Does Strava define elevation gain differently from Zwift? Is this a bug?