IMO is not ideal and makes me question my use of generic combinators in Aeson.
At minimum, this should be documented as a gotcha.

Tested with aeson 1.5.6.0

Start like this

data FooBar = 
    Foo
    | Bar 
    deriving Generic

Things work as expected

import qualified Data.Aeson as A
>>> A.genericToJSON A.defaultOptions Foo 
String "Foo" 

New functionality is requested, new constructor is added, and FooBar becomes

data FooBar = 
    Foo
    | Bar 
    | Baz Int
    deriving Generic

>>> A.genericToJSON A.defaultOptions Foo 
Object (fromList [("tag",String "Foo")])

That IMO is concerning, that breaks compatibility. Adding a new constructor has changed how my old constructors are ToJSON and FromJSON-ed.

There is more (and is possibly related). One would expect allNullaryToStringTag to force the previous behavior (ideally I would not want to do that but let me try)

A.genericToJSON (A.defaultOptions {A.allNullaryToStringTag=True}) Foo
Object (fromList [("tag",String "Foo")])

Nope, what works is:

A.genericToJSON (A.defaultOptions {A.sumEncoding = A.UntaggedValue}) Foo
String "Foo"

Except this will drop tag in the Baz constructor too.
Maybe this has been fixed in newer versions? I could not find a ticket related to this.