@phl thanks for extended info. Indeed, there is a problem with Markdown Plugin.
For example, lists parsing will always try to concatenate
::: blocks. I already added several additional rules to address some list issues.
But, It looks for me now a bit problematic, to have a dozen rules, just to handle parsing result from another plugin.
There is a related issue, https://github.com/NodeBB/NodeBB/issues/3897
As you can see, It's closed, and there is no alternative solution/proposition.
I'm sure, that, if you have blank lines around
::: everything should be fine. But there is no way for now, to ask users to have blank lines around spoiler tags.
Initially I thought about 2 pass parsing. 1 pass - add blank lines around spoiler tokens
::: before Markdown plugin, and 2 pass - apply spoiler rules. But, plugin hooks doesn't support 2 same hooks with different priorities.