Just so we are all clear, Horak only appears in the titles of episodes he appears in. He isn't in the titles this week. When he does appear he's out of alphabetical order at the end (because he is reoccurring). Then Romijn is at the very end with "and" credit. Discovery has cast appear and disappear from the credits all the time. If you are saying that you think he originally was to be credited only in the end titles, like Kyle would be, I see no evidence for that. He's just the only "in between" actor on SNW, whereas on Discovery it was most of the bridge crew.
At least Babs Olusanmokun is also on some kind of fractional deal, he did not appear and was not in the credits for "Children Of The Comet" (which was a real loss to that episode, in my view -- you really want the full cast at that Pike dinner party in your 2nd episode).
I hadn't realized that Horak was the only one out of alphabetical sequence (besides the top three), but it actually supports my point, which is that if the chief engineer was played by a sighted actor, he probably wouldn't have this quasi-recurring status. Again, I have no firsthand knowledge of this particular situation, so who knows, I'm just saying I have firsthand knowledge of many situations like this one, and I never encountered one where some form of bias (conscious or unconscious) wasn't the actual reason the actor had fewer episodes guaranteed, lesser billing, etc. They always say it's a creative decision, and, in my personal experience, that was the truth on zero occasions.
Of course it's possible, but I would be surprised if creative choices were the motivation here. It would be an anomaly. Depressingly, it's just not the way business is done.