I think it is a collision between old and new functionality, part of the problem is that the patterns are used for other things than citation keys. If you were to use [authorsAlpha] for something different than citation keys, the + would probably show up by default. I don’t remember why + was added to “unwanted characters” for citation keys, but I’d guess it caused incompatibility with some setups and is therefore removed by default from all citation keys.
As is, I expect that you have to change your preferences according to Customize the citation key generator - JabRef and remove the + from the list, and it should work as you expect afterward.
Perhaps the documentation for authorsAlpha would benefit from a link to the section of unwanted characters.