Trial chambers can generate small (3x3x1) empty rooms behind walls. Also happens on 23w45a (havent tested other versions).
How to reproduce
Use seed -3955354196018892310 (experimental features turned on)
/execute in minecraft:overworld run tp @s 67.45 -14.00 208.70 -541.11 6.77
That's not really a room; that's the space in front of the entrance door to the entrance room. It often just generates buried. It's more obvious if you place the structure with commands.
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