(DITA 1.3) Changes to @frame attribute on <fig> element aren't saved

Hello everyone,

I'm authoring content in DITA 1.3 in FrameMaker 2020.

If I change the value of the 'frame' attribute on a <fig> element, the value is reset when I save and reopen the file. (The 'frame' attribute adds a border to figures in the output.)

This only seems to happen with the 'frame' attribute.

And it happens whether I change it in either authoring or XML view.

I am not very familiar with creating/customizing structured apps, so I don't know if there is a file I should edit.

Any help would be appreciated.

Thanks in advance!