glossaries package FAQ
As from version 4.0, commands such as
\Glsentryname are robust. (Although, as they aren't expandable, they can’t be used in PDF bookmarks.) For earlier versions of glossaries commands like
\Glsentryname were fragile and therefore need to be protected when used in a moving argument.
As from v4.50, these commands can now expand in PDF bookmarks.
Alternative link: https://www.dickimaw-books.com/faq.php?itemlabel=glsmfirstucfragile