I'm pretty impressed that hlint saw that those were equivalent.

That was me, actually. hlint gave a different suggestion for that line, and I noticed the redundancy.

(That was the only non-hlint thing done, other than adding some modules to "ignore: use camelCase").

Not totally sure why it's failing to build.

Build failures are because of warnings (CI uses -Werror for the pandoc package).
For example:

160
src/Text/Pandoc/Readers/LaTeX.hs:34:1: warning: [-Wunused-imports]
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    The import of ‘<$’ from module ‘Data.Functor’ is redundant
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   |
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34 | import Data.Functor (($>), (<$))
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   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

You can look through the build logs if you follow the Details links for the CI.

I see, that's a bit earlier in the run. For some reason stack and ghcid aren't reporting that warning to me. Normally I fix them all.

You can get different warnings for different compiler versions, so that may be the issue.