Just to let know I had this same issue. I only saw it yesterday (not using ARC that much). Both Roon and ARC were on the latest version on my Apple iPhone and server. Reinstalling ARC on my phone fixed the issue.
Same ARC behaviour since last update. Can use it if I first use offline mode and then turn that off. Though it doesnt show my latest database additions, as if it were disconnected since the last timestamp.
I have the same disconnect between Arc and Roon Server though both are up to date versions. Uninstalling/reinstalling Arc and wiping out downloads is not an acceptable solution.