Using the [details=Summary]This text will be hidden[/details] in a private message results in the Summary drop down appearing correctly, but the hidden text appearing immediately below, and not hidden as it would be in a topic post.
It does, however, work in the markup editor when you are writing the message.
I don’t think the issue is related to PM but rather the formatting of your post. Can you share the markup you’re having trouble with?
Yes as follows:
Finally, you can use the following form to encapsulate longer examples or inclusions to make the discussion easier to follow.
Typing this:
`[details=Summary]This text will be hidden[/details]`
Results in this:
[details=Summary]This text will be hidden[/details]Try adding new lines around the BBCode tags 
Ahh, that works. It did still display correctly in the editor though, so a bug for being inconsistent, if nothing else!
It will be fixed once we switch to a newer markdown engine
We “fix” this by correctly handling details now! [details=Summary]This text will not be hidden[/details]
Because details is a “block” level construct, we never meant to support it inline. That was a bug.