It sounds like you're the victim of what is technically known as "a wanker".
Wankers think they know stuff about imaging, but actually they know stuff-all. Among the nonsense that wankers demand is "The file must be Adobe RGB", or even "The file must be ProPhoto RGB"; "The file must be saved as a TIFF", and in your case "The file must be 16-bit".
Needless to say, wankers are a pain in the ass.
If this wanker engages you for another one of these jobs in the future, just work in 8-bit as normal, then convert it to 16-bit at the end. The wanker will never know.