Reddit-style angle-bracket-exclamation-point spoiler tags do not work on desktop Lemmy or, to my knowledge, any mobile app besides Sync. Lemmy spoiler tags work a little differently:
:::spoiler Text next to the arrow
Text inside the spoiler
which can be multiple lines
* and can contain *formatting*
* and other such niceties
:::
The angle-bracket spoilers also work on the eternity client, as it's just forked from some older reddit client. I made a spoiler oopsy recently with it.