Upstream maintainer here: I could not verify that this still happens with the latest version of the xml indent script. Please verify using the runtime files from https://github.com/chrisbra/vim-xml-ftplugin/
If that is fixed, I suppose the next upload to Debian will come with updated runtime files (it's already fixed in Vims source distribution). If not, please open a new issue at the upstream repository with clear reproduction steps. Thanks, Christian -- Hallo Kakteenzüchter!