What a rabbit hole I fell into!
I was able to reproduce the problem on Circle CI. Basically, we need gcc7 during the build step because epa-ng uses C++14. conda-forge is in the middle of a complete overhaul to make this possible:
https://github.com/bioconda/bioconda-recipes/issues/10771
However I don't know how to enable similar compilers on bioconda yet:
https://github.com/bioconda/bioconda-recipes/issues/9404#issuecomment-432387198