If you post a full repro, i can run it on my 2019. But I see little point in trying to determine the threshold for deferred allocation for every version of SQL Server. It is evident that deferred allocation happens with less than 128 extents on 2016: you have seen this with your own eyes. But trying to determine the cut-off for all versions seems a little pointless. But, again, if you post a full T-SQL repro script, I'm happy to run it on my SQL Server.