Steps to reproduce the issue

Create a Menu Item Alias with Subitems
Create a Menu Module and try to select the Menu Item Alias as Base Item

Expected result

You should be able to select the Menu Item Alias as Base Item

Actual result

You have to switch the Menu Item Alias to another Item Type
Then you can select it as Base item
Then you switch back to Menu Item Alias to have the wished result

System information (as much as possible)

Joomla 3.9.16

Additional comments

Usability fix
Also Affected Menutypes are: URL, Headings, Seperators - they should be selectable as base item in a menu module.