Custom Richtext is not updated through rest API

Custom Richtext is not updated through rest API

Hi
I have a rest API client that I can update all fields of a lead, except a custom richtext field which is not updated.
The funny part is that it returns success but the record is not updated.

I have also double checked the API name and it exactly matches the API name.

Any clues what needs to be done specially for this field?

Ehsan