Something is wrong with your proxy schema. Go into Zotero Connector Preferences -> Proxies and remove all configured proxies, then restart your browser and try opening a proxied website again. The connector will re-add an appropriate entry.

Do you want the library website proxied, though? I don't really see why you'd want that (and yes, it's absolutely possible the parallel URL structure is part of the problem)

Right, but that proxies ieeexplore.ieee.org and not the library and having the library proxied should make no difference on whether ieee or other paywalled sites are correctly proxied.

If you do access the resources through the library, it will automatically send you through the proxy when trying to access them, no need to proxy the library website itself. If you do access them directly, e.g. by clicking the link above, Zotero would handle the redirect through the proxy.

You should restart your browser and manually navigate to https://ieeexplore-ieee-org.ezproxy.lib.utah.edu/document/8594834 , which will redirect you to your library page and ask you to log in. After logging in Zotero will recognise the proxy and in future will redirect you properly from unproxied ieee pages. For other publishers you may choose the option from the Zotero right-click menu to redirect via the proxy and later Zotero will automatically redirect those too.