Anchor cannot be used when page name contains square brackets

We would like to reach anchors in other pages. This does not work if there are square brackets in tho other page's name.
I.e., something like:
DocumentName[Group]#Anchorname
is not recognized as a link in Confluence.

It will also not work when the complete URL is used, for instance:
https://ourconfluence.com/pages/viewpage.action?pageId=123456789#DocumentName[Group]#Anchorname
or
https://ourconfluence.com/pages/viewpage.action?pageId=123456789#DocumentName[Group]-header
to reach a titled section.

We tried different variants. Strange thing is, that I could realize this with exactly one example but it is not reproducible (while the error is reproducible).

As we use square brackets throughout the system this is a bigger issue for us.

Thank you for looking into this.
Mel