Posted Variable Values Being Stripped

I have a client who uses an addon from a different developer (not responding to their needs). The addon is installed on 2 sites with the exact same version of cs-cart (including other addons and schemas).

On site1 everything works fine. There is a separate tab on the order.details page for shipping label info. On this page there are form variables named like 'delivery_type', 'pkg_weight', etc. (not within a separate array container like my_shipping[delivery_type], my_shipping[pkg_weight].. On site1, POSTed data comes through just fine with delivery_type=type1, pkg_weight=4.5.

However, on site2 the variable values are being stripped. I.e. the variables are coming across, but empty. I.e. delivery_type=, pkg_weight=

I had thought that maybe they were being stripped due to fn_trusted_vars() but there's no call to this on either site within this addon. I also thought that a security issue would remove the variable and its value.

The schema tree of the two sites is the same.

Any pointers?