I tried adding it a while ago since I don't exactly have the pleasure of time (deadlines, deadlines, deadlines). Cpanel returned a successful addition, but now all my addon domains are gone. Including the originally affected domain, even though it was "successful" when I tried adding it. Ouch. And the other domains are giving me "account queued" while my originally affected domain is giving me a domain lookup error as usual. The domains still appear to be in the system (you can see them in the subdomains drop-down list) but aren't configured in DNS zones nor are they showing up in the addon domains window either (as mentioned earlier). Adding them again gives me domain wrapper errors on my subdomains <domain>.klswxa.heliohost.org or the addon domain itself.
Hopefully, things will sort themselves out over time. And quickly, too. Sorry if I seem like I'm in a rush. Either way, thanks for all the hard work.