When editing using proxies, File → Export Frame exports a low-quality frame from the proxy sources.
As with other outputs, I would expect the exported frame from to be a full quality render.
I think that’s a very straightforward minor bug (with an easy workaround involving F4!), but if I can provide further info to help diganoise it, please ask.
This does seem like a stopgap rather than a real solution, which surely should be that the export is at full quality even when in proxy mode? I appreciate that this might be rather low priority though!