Just a relatively basic structure generation collision issue. The center structure of the ancient city that I found was almost entirely obstructed by an underwater lake that generated above it. (Also partially a lava lake that generated above, but I didn't take a picture of that one.)
Sorry if this isn't a bug that is worth reporting, since collision is pretty common.
Thank you for your report!
We're tracking this issue in MC-249784, so this ticket is being resolved and linked as a duplicate.
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