Reproducible in singleplayer and multiplayer (commit 45ff58b, 1.12.8). Despite Horologium being up in the sky, the attunement altar fails to activate throughout the entire night period. I added time to speed up the process, but unless the attunement activation happens at a certain time at night, it seems like a bug. Thought I would report just in case. I forgot to label my /forge mods in the video (It is quite lengthy, as I was toying around with the altar and attempting to get Horologium to work - all other constellations worked, apologies for that)

video log

Let me know if you need any more information regarding this, hope I could be of help!

P.S (And yes, I added WorldEdit to speed up the altar building process - I'm lazy)