In case you are using Uniface version > 9.6.05 then the following issue could be causing the empty content:
>RI-3859 - UHTTP only sends Header/Content parameter when using PUT/POST Method
There's no solution available in Uniface 9.6, I'm afraid - except downgrading to 9.6.05 (or lower) and patch X604 (or before). Since 9.6 is not supported anymore it might be a good idea to upgrade to Uniface 9.7.05 or (even better) version 10.3.02.
Apart from the mentioned issue I'm not aware that the Content-Type "application/vnd.api+json" could cause any problems.
I hope this helps.
Kind regards,
Daniel