Upon further testing it turns out that the only one that doesn't vanish is the last alias in the list, and your added alias being the last one was the only one which would not be pruned from the list.
About a decade or two ago, I had a fleeting obsession with unloading excess/uncessary/empty/missing alias/remote files. This meant that if you tried to create more than one empty file with File/New, the editor would create it and then unload all other empty files. It also meant that if, on start up, there were missing files - mIRC would remove all of them except one - just in case you really, really needed that one empty file.
If I remember correctly, someone spotted an issue when doing this with remote files, so it was disabled for them. But I left it in place for aliases.
In retrospect: mildly amusing.
In the name of good sense, I will be changing the next beta to allow you to create, and keep open, as many empty alias files as you want. What a relief!