Apple seems to have changed the format of the string. It was plain xml until now, and it’s now in the binary plist format.
I suspect any old string will be compatible with older systems and the new one should stay compatible with the newer systems.
Now, to convert from the old one to the new one (or the other way around), you could try to convert from one format to another; I don’t know if it’ll work specifically for a setup string, but you may give it a try:
Save your setupstring to a temporary file (the utility converts existing files only, no input stream). Then, use either of these using a shell:
plutil -convert xml1 YourFile.plist
plutil -convert binary1 YourFile.plist
(the former converts to xml (legacy format) and the latter to binary (new format))
Then re-read the file, which will hold the other version. Apply it to the setupstring and cross your fingers.
Of course, you’ll have to first validate the source string: if it’s xml on a system prior to MacOS 26 or it’s a binary string on MacOS 26 or later, you don’t need to do the conversion.