Some code that used htmleditor as an input type in a course format stopped working in 3.6 due to changes that validate the course format settings on every save.
That new validation code seems to assume all return values are single values, and not arrays so throws an error when faced with the return from this input type.
(It's possible this causes issues for any other types that return an array, multi-selects perhaps?)
The error is:
clean_param() can not accept arrays use clean_param_array() instead
It seemed like it should be possible to override this in the format, but attempt to do so got past that error, and then when you returned to the edit page the values had disappeared, at least when following the instructions in deprecation warnings to use editor instead of htmleditor.
Currently working around this by using the deprecated htmleditor and silencing the error.
As I said, this might be fixable in the format, but wasn't sure if that was the intended outcome of the change to add validation