Not sure if it's my specific app or lemmy as a whole, but that spoiler tag isn't working. Sometimes they'll break if you put a space between the tag and your text.
... >! Testing with spaces on both ends !<
... >! Testing with a space at the front!<
... >!Testing with a space at the end !<
... >!Testing with no spaces!<
Looks like reddit spoiler tags just don't work in general