I think there are 2 "bugs" both of which can apparently be fixed with relative ease.
- The OPL file should have used
Context()->normalStrings;afterEND_TEXT. If that is added, the problem seems to render inline (for me, using the icon to render the problem). This is a matter of the XML code being more sensitive to some things than the regular problem rendering code. - The XML generation code in
xml_filterwhich is used to process the inline rendering incorrectly detects something in the answer hash as an array reference, and then runs into a fatal error. I think I have a patch for this which I put in https://github.com/openwebwork/webwork2/pull/1146. Hopefully that patch will make it into a future version of WW, so that this sort of issue will not occur elsewhere.