Some type families don't reduce with :kind!

Sadly, I'm unable to reproduce this without singletons. :(

Observe:

> ghci
GHCi, version 7.10.2: http://www.haskell.org/ghc/  :? for help
Prelude> import Data.Promotion.Prelude   -- singletons-2.0.1
Prelude Data.Promotion.Prelude> :kind! Lookup 'True '[ '(True, True) ]
Lookup 'True '[ '(True, True) ] :: Maybe Bool
= Data.Singletons.Prelude.List.Case_1628035408
    'True
    'True
    'True
    '[]
    (Data.Singletons.Apply (Data.Singletons.Apply (:==$) 'True) 'True)
Prelude Data.Promotion.Prelude> :kind! Lookup 'True '[ '(True, True) ]
Lookup 'True '[ '(True, True) ] :: Maybe Bool
= JustSym1 'True

Note that the exact same command run twice yields different results. I tried peeking at -ddump-tc-trace but got nothing of use.

Trac metadata

Trac field	Value
Version	7.10.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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