Here is the relevant explanation from Mathew Ahrens explaining the issue, the problem is that when old blocks are freed they need to use the old deflation ratio to prevent space accounting over-accounting for freed space.
This means that it would be possible to solve using a “time dependant deflate ratio”, which was not implemented due to lack of time.
In the future, “RAIDZ Expansion improved space accounting” could be implemented, which would resolve the issue, and he thinks “that it shouldn’t be much more work to do this as an extension”
