@adriludwig, I almost always get sleep stages (light, deep and REM), but my wife only gets sleep patterns (awake, restless, and sleep) most of the time, even when wearing my device.
This has been going since she got her first device in March of this year, with three different devices, so it is very clear that the Google-Fitbit Sleep algorithm works much better for some people than it does for others.
Hopefully, Google-Fitbit will eventually release an updated device firmware or app software version that resolves this performance issue, but be advised that it is not required to pay for a Premium account to get sleep stages.
My wife still has a Premium account and never gets sleep stages, but I let my Premium account expire and I still get sleep stages most of the time.
