That _expression_ matches the string of spaces separating the first
Mawhile headline from its tags in many many different ways, and is one
of the type of regular expressions warned about in the (Emacs Lisp
Reference) manual. We shouldn't be generating it, and it is a bug, but
it is in Org, not Emacs.
Ah... Well, this is the reason I posted it here first.
In that case, the bug is present as late as a97f2c1830910d7a239dcbb98e200a6e4038acaa.