Yeah it seems to be just anything on that first line of a post or comment since that's what the apps grab as a preview of the post, same happens on Boost (like Cadeillac said) anytime I see it.
Long as the spoiler code starts anywhere but the first line of the body it seems fine.
Not who you're responding to but - Assuming based off your profile that you're the voyager dev: Just be aware it's not exclusively a voyager problem. Same problem Neon's describing also happens on other apps (Boost for Lemmy for example).