So, turns out that the SSF_* values are wrong since one of the values were missing from the list of constants. For example, SSF_EXIT is supposed to be 1<<7, which is 128...
https://git.do.srb2.org/STJr/SRB2/-/blob/next/src/r_defs.h#L346
...but the actual Lua value is 64 when I tested on next, and when I looked into it, it's because it was missing an entry in SSF_LIST, particularly NOSTEPDOWN. The list should be updated now, though, and since none of this is documented at all in the wiki, it's unlikely it's going to break anything.