the 'spoiler' in spoiler tags actually has to start with a lower case s.
I think it also has to then be followed by at least a space, and then and enter/return/line break
::: spoiler
empty spoiler
:::
Spoiler
Spoiler spoiler
Welp, nope, ok, a spoiler has to have a space, and then some kind of title following the space
... or maybe i am using a mobile app that has slightly different spoiler rendering syntax ... ?