I have the same issue with my Pixel 2 and this was working flawlessly. I have found out that if I have used my fitbit without syncing everyday, this issue occurs. I feel that Fitbit or google have changed something that is causing this. If I connect to the fitbit app everyday on my phone then there is no issue. Plus if I dont sync for a few days, the sleep syncer is messed up and not giving me complete details. I am paying premium so I am expecting to have my sleep readings done properly. Since the specs are saying that I should be able to keep 7 days of data I should be able to sync more than 1 day with no issue. Right now the only help offered is reset everything and connect, uninstall everything and connect. If none of this works you need to buy yourself new laptop or device to connect and sync to it. This is the only solution they offer. And its a workaround not a solution.
Hints for fitbit to reproduce the issue:
Use a alta hr synced with a Pixel 2 for a week and keep syncing everyday
On the Friday leave it on the shelf for the weekend not being used and not synced
Take it back on the sunday and on the Monday morning try to sync with your phone.
This is how I ALWAYS reproduce the issue.
