If you break any block under gravel it all falls and break as if there was slab under it Might be the same thing with sand too idk
I think it has to do with preventing raids for a week after the reset. TNT cannons are disabled, Sand/Gravel slime cannons are disabled by making sand act this way.
Has been fixed in the latest patch. Thank you for reporting =) > TNT will be enabled in 1-2 days (https://www.guildcraft.org/threads/04-12-2017.68896/)