I've made a post and a comment using the spoiler tag. On desktop and mobile Lemmy sites, it works as intended, hiding the text. In three apps I looked at the post & comment on, the spoiler tag doesn't hide the text. The apps I used are Jerboa, Liftoff, and Connect.
I know It's been ages and there's been movies and such, but I'm reading these books to my kids for the first time.... Only up to book three. Boo for spoliers not working in Jerboa.