Well, this is Garmins fault. They don’t send third party activities - which pushes activities to Garmin - to us.
In the meantime - if you have an Strava account - you can connect Strava with Runalyze and let the Zwift activities synchronize to us via Strava.
We hope that Zwift integrate RUNALYZE as a sync option: Please vote here: https://forums.zwift.com/t/synchronization-to-runalyze/13993
Please let us know if you have wishes or have encountered bugs. We give our best to make Runalyze as good as possible for you. - Therefore we would be interested in your suggestions for improvement. In general most questions, problems and ideas are more appropriate to ask in the forum.