Ok now it’s fixed itself so I don’t know
Yes, it’s common for people using mobile platforms because mobile operating systems handle app backgrounding differently than desktop operating systems. tvOS is not “mobile” per se, but it is much more similar to iOS than macOS.
The TLDR for Apple TV, iOS and Android users is it’s good practice to force-close the Zwift app after you save and exit your session.
Powering off your device and rebooting it semi-regularly is also a good idea - not just for Zwift, but in general.