It's not possible to use JS to save a form directly, unless it's backed up by a trusted function in a folder-level script.
Ah, I see. That is strange... What kind of script is this? What's its purpose?
Also, did you write this code, or did someone else?
I am very confident that there is no faults with the script itself, or the form names as I have checked that side of things quite a few times. As I said in the question, the problem seems to arise when the pdf is saved in acrobat DC.
I think you answered your own question... You only installed the folder-level script under Reader, not under Acrobat.
If you look in the JS Console in Acrobat you'll probably see an error message about a function or a variable not being defined, or something similar to that.
Just like the application installation folders are dependent on the product and version of Reader or Acrobat, so are the user application folders. You need to make adjustments in where you place your code according to your product and version. These folders will also be different on different computers Operating Systems.