Assuming I understood what you meant by build with the embed_data_files flag off, I tried.
xxxxx:pandoc-LR xxxxx$ stack build --flag pandoc:-embed_data_files
Building all executables for `pandoc' once. After a successful build of all of them, only specified executables will be rebuilt.
pandoc> configure (lib + exe)
Configuring pandoc-2.10.1...
pandoc> build (lib + exe)
Preprocessing library for pandoc-2.10.1..
Building library for pandoc-2.10.1..
[ 1 of 171] Compiling Text.Pandoc.App.FormatHeuristics
[ 2 of 171] Compiling Text.Pandoc.Asciify
[ 3 of 171] Compiling Text.Pandoc.CSV
[ 4 of 171] Compiling Paths_pandoc
[ 5 of 171] Compiling Text.Pandoc.Emoji
[ 6 of 171] Compiling Text.Pandoc.Extensions
<command line>: can't load .so/.DLL for: /Users/lrosenth/.stack/snapshots/x86_64-osx/a4b9df8230b44b4d1656bd3420d752e1440163e4e9d142f613e0f472365ab993/8.6.5/lib/x86_64-osx-ghc-8.6.5/libHScommonmark-pandoc-0.2.0.0-D01mbkJUemk7h72RuTUjAh-ghc8.6.5.dylib (dlopen(/Users/lrosenth/.stack/snapshots/x86_64-osx/a4b9df8230b44b4d1656bd3420d752e1440163e4e9d142f613e0f472365ab993/8.6.5/lib/x86_64-osx-ghc-8.6.5/libHScommonmark-pandoc-0.2.0.0-D01mbkJUemk7h72RuTUjAh-ghc8.6.5.dylib, 5): Symbol not found: _pandoczmtypeszm1zi21zmLb7evYKD6jv46JnhbWrkOK_TextziPandocziBuilder_normalizzeTableFootzunormalizzeRowszq_closure
Referenced from: /Users/lrosenth/.stack/snapshots/x86_64-osx/a4b9df8230b44b4d1656bd3420d752e1440163e4e9d142f613e0f472365ab993/8.6.5/lib/x86_64-osx-ghc-8.6.5/libHScommonmark-pandoc-0.2.0.0-D01mbkJUemk7h72RuTUjAh-ghc8.6.5.dylib
Expected in: /Users/lrosenth/.stack/snapshots/x86_64-osx/0bf2dc0c6fb03a8a3c3fd1819c92943191347f522011fdc733a961a0bb6b76dc/8.6.5/lib/x86_64-osx-ghc-8.6.5/libHSpandoc-types-1.21-Lb7evYKD6jv46JnhbWrkOK-ghc8.6.5.dylib
in /Users/lrosenth/.stack/snapshots/x86_64-osx/a4b9df8230b44b4d1656bd3420d752e1440163e4e9d142f613e0f472365ab993/8.6.5/lib/x86_64-osx-ghc-8.6.5/libHScommonmark-pandoc-0.2.0.0-D01mbkJUemk7h72RuTUjAh-ghc8.6.5.dylib)
-- While building package pandoc-2.10.1 using:
/Users/lrosenth/.stack/setup-exe-cache/x86_64-osx/Cabal-simple_mPHDZzAJ_2.4.0.1_ghc-8.6.5 --builddir=.stack-work/dist/x86_64-osx/Cabal-2.4.0.1 build lib:pandoc exe:pandoc --ghc-options " -fdiagnostics-color=always"
Process exited with code: ExitFailure 1