Dismounting an entity is left-shift by default, do you usually change the key bind to space? Loading the snapshot may have just reset your key binds or unbound some keys.
I don’t work for or are associated with Mojang, I’m just trying to help out.
here’s a video of the issue, for some reason I can’t upload it so here is a youtube link to it instead.
[blockCard]
Can confirm that Sulfur Cubes are unable to spawn in peaceful mode in 26.2 Snapshot 3.