I tested further on a different system using the same seed as above, and the fortresses did indeed generate. I didn't think to check whether this could be an issue with mods instead of the vanilla, but after the second attempt I believe this was caused by Fabric, not Minecraft. However, my issue may be different from the reporter, so I'll defer to Steve.
Nether fortresses didn't generate for me either. Seed 7066370557625904277
/locate Fortress didn't output a location after nearly 2 minutes of processing
Additional behavior: When walking from a full block onto such a non-full block adjacent to a slab, the player is pushed to the non-full block.
Video of behavior: https://youtu.be/fLWGsFpK2Kk