Using a fresh 1.21.1 instance with only the lost cities mod installed, I noticed the stairs are not rotated correctly. In the F3 screen the #lostcities:rotatable tag is also missing when looking at a stair block. Using /neoforge tags minecraft:block get lostcities:rotatable also result in unknown tag errors.

Some further investigation reveals that in 1.21 and above, tags/blocks got renamed tags/block: https://github.com/neoforged/.github/blob/main/primers/1.21/index.md#depluralizing-registry-and-tag-folders. However, the generated resources still use the plural folder name here: https://github.com/McJtyMods/LostCities/tree/1.21_neo/src/generated/resources/data/lostcities/tags/blocks, which is why those tags are not loaded by the game.

Renaming tags/blocks to tags/block should fix the issue.

Thanks for figuring this out. I will fix this