Given those two options, I think I’d pick “A single spoiler, but the selected content will be forced into its own block.”
I can’t really give a use-case based reason for this; I think the forced-block behavior would still be buggy.
But the forced-block option feels “less buggy” to me because it “only” affects how the spoiler looks: adding an extra line break at the beginning and end of the spoiler.
Having multiple unlinked spoilers affects how the spoiler works. Readers would have to click as many as three times to reveal the entire spoiler: once for the leading inline spoiler, then again for N block spoilers, and then again in the trailing inline spoiler.
With forced-block spoilers, it becomes a cosmetic Hard Bug, the sort of thing that maybe nobody will ever work on.