This happened to some people and there were several posts and at least one thread about it. Maybe this happens only with older downloads that have existed for a while and it might have been fixed in the meantime for new downloads - I am not sure if that was 100% successful but there seems to have been an attempt. It happened to me in the past but has not happened again for considerable time after I deleted all downloads and started again after a “tentative fix”.
Maybe it helps to start ARC while the phone is on your local wifi and disabling offline mode, to give ARC a chance to resync the content. But if not, then I don’t think there is a solution short of resetting the ARC app (which will also delete all downloads)
The thread were this was reported in Early Access testing and it was “tentatively fixed”, and I have not had the issue since: