That is how it works on Reddit. Lemmy has its own entirely incompatible spoiler tag system which works like this:
Text next to the arrow
Text inside the spoiler
which can be multiple lines and can contain *formatting*
Which produces this:
Text next to the arrow
Text inside the spoiler
which can be multiple lines and can contain formatting
Reddit style spoilers that look like >!this!< don't work on desktop or, as far as I know, any mobile app besides Sync.