Probably the issue isn't actually of this app, but of fitbit app itself, however sometimes the recording stops during the night, while the watchapp seems to still work and so the companion according to the logs (sleep-logs.json), but Sleep as Android doesn't receive the data for sure.
According to a forum report it seems that this is due to a latest update of fitbit app though: https://forum.urbandroid.org/t/fitbit-ionic-versa-beta-integration/3035/355
Probably the issue isn't actually of this app, but of fitbit app itself, however sometimes the recording stops during the night, while the watchapp seems to still work and so the companion according to the logs (sleep-logs.json), but Sleep as Android doesn't receive the data for sure.
According to a forum report it seems that this is due to a latest update of fitbit app though: https://forum.urbandroid.org/t/fitbit-ionic-versa-beta-integration/3035/355