When a text field's format is set to "HTML" (typically the "Original Submission" field of a new artifact), it is not possible to change its format from "HTML" to "Text" via the REST API. On the other hand, when a text field is formated in "Text", it is indeed possible to switch its format from "Text" to "HTML" via the REST API.

N.B. the user who reported this issue said that it's not possible to change the field's format from "HTML" to "Text" while changing its contents at the same time. In my tests, though, I confirmed that it's not possible to change the field's format from HTML to Text even if one does not try to change the field's contents at the same time. It has to do with the format change, not with the user also updating the field's contents. And the other way round, when one tries to change the field's format from "Text" to "HTML" via the REST API, it can be done successfully even if updating the field's contents at the same time.