I think this is a parsoid bug. Here's a description from a Flow user's perspective:
If I type/paste this text into a Flow post:
See the link for details (https://www.google.com/link)
Then the closing-bracket is included in the a href (it links to [https://www.google.com/link)]
Version: unspecified
Severity: normal
See Also:
https://bugzilla.wikimedia.org/show_bug.cgi?id=58054