Texas-Mark
Beneath the gated room, there is a pair of tiles (shown in pic below) that becomes active at some point. They are not there in my game; I just don't remember if I went through them from the top or from the bottom. The video shows that player going through the gates and then going down through the pair of secret tiles.
PIC: https://ibb.co/tXxRPdX
VIDEO: https://youtu.be/WcnxDNOHQJU
The pair is definitely not active the first time you walk along that long horizontal hallway, but something odd happens when you break into the secret room with the treasure chest. A new block appears on the left of the treasure chest room. Perhaps the two tiles I outlined might become active at the moment.
It's a long shot, but perhaps worth checking.
