I wasn't able to reproduce your issue, however I don't have exactly the same setup. But I did find a possible timing issue in DuetWebControl that may explain your issue. Please download upload file dwc.js.gz from https://www.dropbox.com/s/qlk90c2pa9z5sch/dwc.js.gz?dl=0 and upload it through the Settings/General page of Duet Web Control.
[Note to self and chrishamm: the change is in line 1411 of jwc.js. Was:
if (fileInfo.filament.length > 0) {
Changed to:
if (fileInfo.filament.length > 0 && status.extrRaw.length > 0) {
]