Yes, it is well documented and many people have observed that the powerwalls will raise the line frequency to shut down the inverters when they are at 100%. However, the question that was asked here is a little different and we had another thread that was discussing this, but we never really came to a conclusion there.
The other concern is that each powerwall can charge at up to 5kW, so what happens when you are off grid and you have, say 2 powerwalls, but your solar system can produce 15kW. The house will use some of that and the powerwalls can absorb up to 10kW, but there may be extra power that can’t be used. Normally that would be sent back to the grid, but when you are off grid that can’t happen. One member said that in this case the entire system will shut down and your home will lose power.