This is not a major issue that effects core functionality, probably just more of a slight annoyance and worth knowing if you are about to enter a number of aliases manually:
If there are multiple domains, when adding an ADDRESS (email alias) the first time;
the drop down box domain selector shows “All domains (*)” as the default;
if you create an alias on a single domain by selecting that domain, it adds it correctly (ie not as a wildcard).
If you then go back to Add another Address, the drop down selector shows the domain used in the previous addition (with a tick next to that domain if you drop it down);
If you leave that as the choice, the added alias becomes a * Wildcard entry - not the domain highlighted in the selector.
To achieve using that single domain again, you need to drop down, de-select the domain, and re-select it.
I’ve tested and reproduced in Firefox and Chrome.