I’ve been trying to fight Yogg-Saron in Ulduar both last week and this week, but both times I cannot get past the Ancient Gate of the Keepers, which remains Locked despite all web documentation I found suggesting it should automatically unlock after defeating Mimiron, Freya, Thorim, and Hodir.
I recall having this happen during WoD or Legion because I had opted out of one of the earlier mini-bosses, maybe it was the Assembly of Iron at the left side of the Antechamber. I don’t fully recall.
The game design of raids from vanilla through Wrath was pretty linear. For instance, Majordomo Exectutus won’t even appear until all prior bosses are slain, and Black Temple is thoroughly gated in a similar fashion.
So, my best guess, is to backtrack and make sure you have done everything except Algalon. If you missed any of the prior bosses, that is the thing causing the problem. If nothing was missed, it is definitely a bug.
Edit: I just did a quick test of Ulduar with a lvl 70 alt. What I noticed was that some of the fights seemed to glitch, the target npc would become untargetable for a split second and regain some very small amount of health before the fight continued. I saw “immune” flash up a few times along with the usual numerical damage figures. Not sure what they were immune to, never saw that before, but it makes me think that something has changed in the code that deals with combat. Yogg-Saron seemed especially glitchy. Yogg-Saron also seemed a good deal more powerful than a level 35 raid boss should be in a 10-man normal raid, Arthas in ICC 25-man Heroic was a cakewalk, but not Yogg-Saron on 10-man normal.