Strongholds that are generated by using the command /place can't be encased in stones or deepslates.

How to reproduce:

1.Create a new world (World Type:Default).

2.Input the command /place structure minecraft:stronghold ~ ~ ~

3.Change to Spectator Mode and fly down until you see the stronghold.

4.You will see the stronghold that isn't be encased in stones or deepslates.