Space is one of the rides where they have the FP sign covered because they typically use an “alternative access” entrance for those eligible to use the FP line. You see the same thing at rides such as EE, JC, Safari, Kali, and a few others. They are generally places where the FP line and standby line couldn’t be used given spacing or congestion at the entrances, depending on the configuration of the line/entrance.
Couple of examples, this article from March, you can see the Space FP sign covered in the article below:
https://www.disneyfoodblog.com/2021/03/19/photos-space-mountain-evacuated-in-magic-kingdom
And also here’s a screenshot from
this video from last summer after reopening:
View attachment 578667
This topic has come up a few times at a few different rides because it gets noticed and people think it’s new, although many/most of the rides that are like this have had FP signs covered since reopening.