We have taken the EXTSSLIT-1879 bug into work; unfortunately, we can't reproduce the issue with 100% confidence.
In our test environment, we can reproduce the blank confirmation issue very-very-very rarely. At the same time, all the necessary text appears on the confirmation screen after a second just after the page reloads. Also, we do not observe actual results with different certificates on the domain and subdomain. Likely, we have found another issue, but all these mean we still have an issue with reproducing the bug on our side.
Anyway, we would like to find and fix the root cause that happens more often (based on the number of reports in this thread, Facebook, via Plesk Support); or to confirm that there is only one bug and it was already found.
So we are looking for people who are experiencing this issue on their servers right now (the black confirmation banner is shown, not disappearing automatically in a minute; where different certificates are created instead of one wildcard certificate), to invite them to contact Plesk Support. Even if you do not have direct support from Plesk, let me know and I will create a ticket behalf you (I just need to know your email address, you can send it to me directly through the forum). The ticket will be passed to an engineer/developer on duty to investigate the root cause on your server (where the SSL It! has failed to reissue a certificate correctly).
If you can demonstrate/reproduce the issue on your server quickly, without the necessary wait for 60 days, let me also know, and I will help you contact Plesk Support as well.