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
I have three accounts, and at least two of them are experiencing this issue. On one machine, the primary account is logged out, and I'm shown the other two accounts. Logging back in with the primary account works fine, but the issue reappears within a couple of days, even if opting to "Keep me signed in."
On another machine, this happens nearly every day.
Launcher v 2.1.5964, Bootstrap v 601, UI v 1314
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