Tittle is pretty much self explanatory. Probably happens to above Y=256 but couldn't test.
Basically the extended Y levels in 1.18 are ignored.

BlueMap: v1.7.2 for 1.18 Fabric

Please provide more info about how to reproduce your issue. (Contribution Guidelines)
I tested the extended world-heights and they worked fine for me :)

Keep in mind that BlueMap doesn't render caves below a configured height by default.

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Oh, I thought the default height for 1.18 would be changed as vanilla changed them, but setting it manually fixed the issue.