When mousing over spoiler markup, the text underneath it shows temporarily, disappearing when you move your mouse cursor away from it.
Additionally, you can click it to make it keep showing even after you move your mouse away (a second click will hide it again).
To demonstrate, here's some example spoiler text:
Clicking this text should make it continue to show even after you move your mouse away.
While this clicking behavior works fine in posts, this doesn't seem to work in user profiles. Here's an example profile which contains spoiler markup to demonstrate this issue; unlike the spoiler text above, clicking the spoiler text in the profile will not make it continue to show after the mouse is moved away.
It appears that user profiles use an older, forked version of the CSS (e.g. link titles are still not underlined despite them being changed to be so in posts recently), and the clicking behavior hasn't made it there yet.
Reproduced in Google Chrome stable (version 79). This may not be reproducible in mobile browsers, since many of them implement a behavior whereby tapping a specific element will make the page think the "cursor" is always on top of it, masking out the effects of this bug. It can definitely be reproduced in a desktop browser on a non-touchscreen machine.