I think adding the fix to a package in backports is a good compromise. I wouldn't even suggest necessarily upgrading PHP to a newer version, just fixing this obvious bug. I don't disagree with your concern that changing this behavior now may affect people that have already had to work around it, but conversely I think it's important that it be fixed specifically because this is an LTS release. People running this release may not otherwise be able to upgrade to a fixed version for at least 2 years.
I'd certainly be happy to help out with the backport in some way, especially regarding testing, but I'm not very familiar with deb packaging.
I think adding the fix to a package in backports is a good compromise. I wouldn't even suggest necessarily upgrading PHP to a newer version, just fixing this obvious bug. I don't disagree with your concern that changing this behavior now may affect people that have already had to work around it, but conversely I think it's important that it be fixed specifically because this is an LTS release. People running this release may not otherwise be able to upgrade to a fixed version for at least 2 years.
I'd certainly be happy to help out with the backport in some way, especially regarding testing, but I'm not very familiar with deb packaging.