Minecraft – What dyes are now available in Minecraft

MCEdit can to this, according to this forum post. You can replace all blocks of a specific ID with another (e.g. Air).

Press Ctrl-A, and select your entire map. Then press 4, then R. Then put in IDs.

According to the wiki:

Empty prison cells: A corridor with one wall partially made of iron bars and with two iron doors (in an "open" position, with no buttons). Beyond each iron door are cells with an iron bar wall between them. An exit is opposite the entrance. Maximum five per stronghold.

Your picture definitely matches the description of the Minecraft vanilla stronghold prison cell.

What I first thought:
I first thought of this issue about the chests, cobwebs, etc not spawning could be a generation error. But in my understanding, that's not how generation errors work. They work as if they cut segments of the stronghold — for example, if half of the prison cell room was missing. They shouldn't affect specific blocks in the stronghold.

What I later realized:
Then I got to thinking, and this closely resembles what I've seen in previous mod packs. You could be experiencing errors in the mod pack that prevent the items from spawning. A sort of a conflict might arise between mod pack and the dungeon generation. One possibility is that it's only on your version of the mod pack, and something went wrong during the installation process.

What also could have happened:
It could also be the actual mod pack itself being entirely broken, not uncommon with these brand-new packs. I can't pinpoint the problem specifically; you would have to send a bug report to the devs and hope they fix it. I am almost 100% certain this was unintentional. Good luck.

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