Cabal sdist concatenates names of conditional Main module names

A minimal Cabal package description:
~~~~
Cabal-Version: 2.4
Name: cabal-cond-main
Version: 0.1

Flag alternative
Description: Alternative implementation
Default: False

Executable cabal-cond-main
If flag(alternative)
Main-Is: MainA.hs
Else
Main-Is: MainB.hs
~~~~

I run
~~~~
$ cabal-3.10 sdist
Error: cabal-3.10: MainA.hsMainB.hs doesn't exist
~~~~

I guess there is some `Monoid` instances screwed up.